I thought over the comments a bit.
Yes... QuickTime Player is the one player I think none of us have tried, and historically, I've had better results with any other player. Nor have any of us tried Windows Media Player.
I use MPlayer2, I think Gonbe uses Media Player Classic Home Cinema, and I would bet that somewhere along the line, Videolan was tested. These work fine.
A lot of us over on my IRC network use Matroska for our main format, we encode everything with x264 and qaac or mainconcept, then put the file in an MKV container. I don't like to use the MKV container unless I need one of its features, so I didn't - Robotech, which is single audio and single video, is encoded with x264 and mainconcept, and muxed in an mp4 (QuickTime) container. So yes, theoretically QuickTime should be able to handle it; but practically, my experience with Apple has been mixed.
The video settings, branded into the video track by the encoding software, are as follows:
x264 - core 129 r2230 1cffe9f - H.264/MPEG-4 AVC codec - Copyleft 2003-2012 - http://www.videolan.org/x264.html - options: cabac=1 ref=16 deblock=1:0:0 analyse=0x3:0x133 me=umh subme=10 psy=1 psy_rd=1.10:0.00 mixed_ref=1 me_range=40 chroma_me=1 trellis=2 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=0 chroma_qp_offset=-4 threads=8 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=16 b_pyramid=2 b_adapt=2 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=288 keyint_min=23 scenecut=40 intra_refresh=0 rc_lookahead=60 rc=crf mbtree=1 crf=18.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
I used 8-bit color depth as, while using 10-bit allows for a comparable quality of video in a smaller file, many players do have issue with it still.
The file is encoded at High Profile, Level 5.0; could this be the source of problems as well? We've never had any issue with it but some players don't seem to like 5.0 level H.264 encoding. (Again - the ones we all use handle it just fine; props to the libavcodec folks.)