A common problem I see is when VMotion fails at the 10% mark with a timeout. You might see a dialog popup which says-
Operation timed out
Tasks: A general system error occurred:
Failed waiting for data. Error 16. Invalid argument
The VMware Knowledgebase writers have assembled a great list of possible reasons for this problem and I [...]

A few customer are hitting a problem with Virtualcenter 2.5 when they use non-default ports for WebAccess.
There is a published KB article on there here: http://kb.vmware.com/kb/1003915
When you try to log in to VMware Virtual Infrastructure WebAccess when a custom HTTP port has been specified (whether during installation, before an upgrade, or after), the error “Web [...]