I played the same bit of video like 20 times in a row, that music is going to resonate in my head for days.
Yeah I think soon I will can sing this song by heart

Edit:
For science! I tried changing the renderer in MPC output settings as well as subtitles settings. I got some interesting results.
I quickly found out that I could get stable framerate (almost, it dropped to 59, 58 fps a couple times) on these troublesome dynamic subtitles by using these settings:

Instead of the default ones:

The texture size at 720p did not represent a noticeable loss on fullscreen (and I have a 1080p display).
I could not find a fix to the high memory usage, but I did notice that different output renderers had slightly different memory performance. I did not make enough runs to call this a truly reliable "scientific" test, but (always on a 720p window):
-VMR-9 (renderless) peaks to about 900 Mb.
-EVR Custom Presenter peaks barely over 900 Mb, and not always gets over 900.
-Haali has the lowest memory usage of all, doesn't reach 600 Mb, but it also had flashing subtitles on my settings
-madVR seems to hit a "limit" at 910 Mb, but always reaches that level.

Something noteworthy: Disabling "Allow animation when buffering" on MPC Subtitles options allows to free up memory much quicker.
I recalled one of these had a higher memory usage so I played the ending with all renderers again but with subtitle textures set to 1920x1080. It would seem Haali uses a lot more memory (even when the window size was 1280x720) with higher subtitle textures. I tried getting to an optimal setting with the Haali renderer with no avail, as it requires "Allow animation when buffering" enabled so subtitles are constantly visible.

Using EVR Custom Presenter and the subtitle settings I posted before I got to this performance graph and a stable framerate all along (on a 1280x720 window). Maximum memory usage was 906 Mb.

Yes, I played the same bit of video like 20 times in a row, that music is going to resonate in my head for days.

You are right. To reproduce memory use bug I need right this "problem" file what you have.

Putting it on clearer terms: I have this video sequence https://www.youtube.com/watch?v=tUS7gSv2ZxE
On "encoding A" and "encoding B".
Encoding A performs excellently on both hard drives.
Encoding B has very poor performance on Seagate hard drive, slightly less bad on WD hard drive.

My internet connection isn not exactly fast for uploads, so uploading a ~300Mb video file for the final 1:30 minutes isn not exactly profitable, and if I re-encoded the troublesome bit and sent it I would lose the particular encoding that triggers this issue.
As I obtained the video file on a "t0rr3nt" site, I'm not entirely sure if I'm allowed to post a such a link here, in addition to the fact I have yet to find a "Forum Rules" section xP
I would PM you but I have found no way of doing so either so... what to do?

I will try to upload the video later today, though.

What filter are you using for subtitles?

Well, this is embarrassing. I have no idea. Overall, video decoding isn't my specialty, I usually fiddle with player settings until I get it working as I want to.

If I understood Media Player Classic's tooltip's correctly, I'm using its internal subtitle renderer, and EVR Custom Pres. for Direct Show video.
I think however, that the MemoryLeaksDetails mod.zip fixed the issue, as I played both the proportionally biggest video I have (1.22Gb for 24 min! 1080p + 3 audio streams, 2 of them in flac lol) and a 51 min 720p video with no issues. Both had ASS (talk about an unlucky acronym) lossless compression subtitles.
Memory usage for Media Player Classic didn't even reach 600 Mb when playing the 1080p video @ 60fps.

P.S.: I should note that I've set all 4 profiles to the same settings.

Show your log file after error occurs again.

Download: SVPMgr_MemoryLeaksDetails_mod.zip

It is high value of memory use at your settings.

From log I see it took 17 minutes from start playing after rewind to used memory grown to 1170 MB:

I met used memory growth when used wrong subtitle filter.
1. What filter are you using for subtitles?
2. Show please used memory graph from SVP performance graphs window while error appears again?

==============
Original Post below
==============
But first, bear with me while I attempt to put in words how grateful I am for this stunningly amazing piece of software. If you ask me, SVP is a prime example of hardworking independent developers pulling out freeware with capabilities commercial software can't even dream of.
Why such a difference? Because software such as SVP is intended primarily to work, not to sell. And that's why I love the thing. It's efficient, unlike most of everything you see out there.
What really moves me is all the effort the SVP developers put into it. I'm not exactly trained on coding, I can barely do some kiddie-level scripting, but just using the UI, reading the documentation and logs, and most importantly: The results. Steady 60 fps on 720p and just 30% cpu load?! YES PLEASE.

Of course, no software is perfect. Yesterday, I got a warning sign while watching an anime episode. I didn't take note of what it said, mostly because of the shock I think, it was the first unexpected issue I ever had with SVP after all.
Fortunately the same message is on the attached errorlog, apparently it said this: "Error: Warning: there's a memory leak in the video player: 1170 MB
To prevent the closing of the player due to lack of memory a script restarted improve smoothness."
Gotta love "good guy SVP" xD
"Detects memory leak. Fixes, warns user and doesn't even break playback."

My 1280x720@24 profile is as follows:
Frames interpolation mode: "2m" (min artifacts)
SVP shader: 2. Sharp (for anime)
Motion vectors interpolation mode (grayed out for some reason): Bilinear
Target frame rate: To screen refresh rate (default)
Motion vectors grid: 32 px. Large 0
Decrease grid step: By two with global refinement
Search radius: Small and fast
Motion vectors precision: Two pixels
Wide search: Disable
Artifacts masking: Strongest
Processing of scene changes: Repeat frame (default)
Decrease frame size: Disable (default)
btw, I've extensively tested these settings and I dare to say it works about perfectly with not just anime, but with live action videos as well.

My system specs:
AMD Phenom II 955 (x4 @ 3.2 Ghz)
Gigabyte 870A-USB3
Sapphire Radeon HD 4890 Vapor-X
2x G-Skill F3-12800CL8-2GBTD (2x2Gb 1333Mhz)

Additionally, the video-related MediaInfo on the anime episode I was watching during the memory leak. Not sure if it'll be of any help but, best to be sure.
[container and general information]
Writing application: mkvmerge v5.0.1 ('Es ist Sommer') built on Oct 9 2011 11:55:43
Writing library: libebml v1.2.2 + libmatroska 1.3.0
[first video stream]
ID: 1
Format: AVC
Format/Info: Advanced Video Codec
Format profile: High 10@L5.0
Format settings, CABAC : Yes
Format settings, ReFrames: 16 frames
Muxing mode: Header stripping
Codec ID: V_MPEG4/ISO/AVC
Duration: 23mn 39s
Width: 1 280 pixels
Height: 720 pixels
Display aspect ratio : 16:9
Frame rate mode: Constant
Frame rate: 23.976 fps
Color space: YUV
Chroma subsampling : 4:2:0
Bit depth: 10 bits
Scan type: Progressive
Title: H.264
Writing library: x264 core 125 r2200+677M 3a754e7
Encoding settings: cabac=1 / ref=16 / deblock=1:1:1 / analyse=0x3:0x133 / me=umh / subme=10 / psy=1 / fade_compensate=0.80 / psy_rd=0.92:0.02 / mixed_ref=1 / me_range=24 / chroma_me=1 / trellis=2 / 8x8dct=1 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=-3 / threads=12 / lookahead_threads=2 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / fgo=0 / bframes=10 / b_pyramid=2 / b_adapt=2 / b_bias=0 / direct=3 / weightb=1 / open_gop=0 / weightp=2 / keyint=250 / keyint_min=23 / scenecut=40 / intra_refresh=0 / rc_lookahead=60 / rc=crf / mbtree=1 / crf=17.2000 / qcomp=0.60 / qpmin=0 / qpmax=81 / qpstep=4 / ip_ratio=1.40 / aq=3:0.92
Default: Yes
Forced: No
Matrix coefficients: BT.709

Apologies for the gigantic post, I didn't find a "compacting spoiler tag" so I couldn't do much about the wall of text.