Or: When is 5296 > 5299?
Many people who managed to get a hold of the leaked IE7 7.0.5299.0 build may be wondering whether they should uninstall it and switch to the official IE7 Beta2 Preview, which is version 7.0.5296.0. On the surface it seems like the 5299 build would be newer, and therefore better, but this is not the case:
The above illustration (and the following description) is a simplification of the release process, but accurate enough. The Working branch represents the branch that IE developers use daily to check-in new features and bug fixes, and as you can see there's a lot of activity going on down there. The Beta 2 Preview branch was forked at a certain point and only select bug fixes were ported into it. This is the part of the process where we "lock down" to reduce the risk of destabilizing changes. During this period we only take critical bug fixes that we are confident will not cause additional problems.
Shortly after we forked we received a code integration from the main Windows branch. IE7 development is happening in parallel with Windows development, so we periodically merge in new code from other teams which may directly or indirectly affect IE7. As with all changes this has the potential to destabilize IE7, particularly downlevel as most people working on Windows Vista do not need to worry about how their changes run on XP. This code integration is what actually bumped up the build number to 5299, so the leaked build is from some arbitrary point in the middle of that churn.
So, to recap, you shouldn't use the leaked build because: