I was looking into a CRM workflow issue where the workflow doesn't proceed at a "wait until task" step. When the dependent task status has changed to "complete" the workflow should continue, but instead it just hangs.
I did a few tests and managed to reproduce the problem by completing the task as soon as possible after it is created by workflow. Delaying the approval doesn't cause any problems. I managed to locate a hotfix from Microsoft: http://support.microsoft.com/kb/951919
Installing it resolved the issue.