I just encountered a nasty bug in Rational Team Concert 5.0 where, if workspaces or components were created in RTC 4.x, they component couldn’t be found (error 400). You could create a new workspace, but any changes you had pending couldn’t be viewed, and neither could changeset history and the like.
After opening a PMR with IBM, I was pointed to this link, which has a fix. Edit: Even though the title specifies 4.0.6, IBM’s technote now specified RTC 4.x.
I’m glad that the RTC developers were able to fix this quickly and even provide an unofficial fix before a real technote has been published.