Currently I am working on a project with a quite complex sales process and part of it is managed by workflows (for flexibility) and to be able to do what is needed, I have developed several custom workflow activities.
However, today I had just installed some stuff and was going to review one of the workflows, and the CRM GUI crashed with the Exception "System.NullReferenceException" in a really nasty way when I tried to open it. By looking at the stack trace I got a feeling it had something to do with my custom activities so I just reregistered them and then everything started working again. So if you come across this error, this might be the problem. I couldn't find any reference to this error on the net why I thought it might be a good idea to blog about it.
Microsoft Dynamics CRM Architect