QT 7.4.5 did **not** fix the problem with some .mp4s

Question

It has been 3 versions now - 7.4, 7.4.1, 7.4.5 - since Apple broke the ability of QuickTime Player to play some .mp4 files. Apple says it is because they have some kind of malformed headers, yet they play fine in iTunes. So why can't QT Player play them, and some people report they have problems when put into Keynote?
Apple has broken functionality for lots of users. Time and again this bug has been brought to their attention. It apparently is a design decision not to fix this, and it is a big disappointment.

Answer 1

The story so far:
If you are having problems playing MPEG4 video in Quicktime 7.4 (and Apple are aware that some users have had such problems) you should carefully check in
/Library/Quicktime and
/Library/Internet Plug-ins and
Home/Library/Quicktime
to check whether you have any third party plug-ins that are not completely up to date.
Current versions of Flip4Mac and DiVX and Perian do NOT cause failure alone or in combination. However the one SPECIFIC extension that ALWAYS causes the failure alone or in groups is 3ivxMediaImp. All other 3ivx extensions DO work. Remove only that ONE SPECIFIC extension.
Please try this and see if it fixes your problem with QT Player, the QT Firefox Plugin, and iTunes not playing MPEG4 files (including mp4 files).
Older 3ivx and DivX components are often incompatible with new QuickTime versions and are, in many cases, obviated by component packs like Perian.
This is not the only explanation for issues with MPEG4 file playback in QuickTime 7.4 and later, however. The issue with videos from democracynow.org and others, for instance, is the result of that content not complying with MPEG4 standards (namely that they use bogus track dimensions). QuickTime had previously ignored invalid track dimensions but this was changed in order to support properly authored anamorphic content.
The recent update to QT 7.4.1 fixed a number of issues but some problems are unavoidable when working with non-standards compliant content. The standards are important for many reasons including keeping hackers from manipulating improperly coded web pages.
A further problem was the fact that the developers of iSquint conceded that in many cases, the files do not play because they have been created in a non-standard way (the tool used to create them was broken). They have therefore included a Debreaker as a part of the latest version of iSquint. If your files are of this type, then you can run them through this app without having to downgrade QuickTime. This could save you a lot of hassle!
http://www.isquint.org/getit.php
However, in other cases, there is a workaround (also the basis for why those files will still play in iTunes): there are a few ways to trick QuickTime into ignoring the invalid track data. For instance, rename the files .3GP and they'll show up just fine.
Post back if you want a list of the plug-ins you should have in order to play pretty much anything in Quicktime.
The recent (mostly security) update to Quicktime 7.4.5 has not changed the situation as described.