As I am working very much with subversion snapshots and unstable stuff, there is a basic rule I have noticed:
Never check out code on Monday mornings.
Why is that? Let us assume an open source project. Most people have hacked code and committed their changes to the repository. So new features were added and maybe and things got broken. So there is a high risk on breakage. But this is nothing to worry about. Most bugs are fixed and checked in during the week. Checking out code on Wednesday or Thursday is to be considered much more safe.
A word of warning: Do not use such snapshots in a real world environment as there are too many unknown side effects for a productive system, except you really know what you’re doing…