Here is another strange but interesting issue, I faced almost a month back. One of my acquaintances was trying to import a CRM Organization [They exported the database and were importing the database into an existing CRM installation] which was using a different domain. Everything was going fine until he reached to the point where you are supposed to map existing users with users from destination AD. The moment he reached that stage he was running into an Exception in Deployment Manager add-in [See the screenshot below].

After trying to troubleshoot this problem for a while, I found few pointers regarding empty domainname field in SystemUserbase table. Upon examination of the SystemUserbase table, we found that indeed there were few records with empty domainname field in there. Now INTEGRATION and SYSTEM are valid users of CRM and they do not have domainname mapped to them however, for every other record there must be an associated user from AD i.e. domainname field must be populated. In our case, there were few other users, which had empty domainname field. We filled in those fields with valid domain\username information and voila, import went just fine.

I recently observed a scenario where a CRM 4.0 database was upgreaded to CRM 2011 and after upgrade process uses were not able to perform any kind of operation within CRM. The exception that we found was :

A potentially dangerous Request.Form value was detected from the client (crmFormSubmitXml="<iteasy_postcode><it...").;Hash='1073421037'

This is because ASP.Net 4.0 now has a different behvaior and it inspects all request data. Rather that being the cookies, urls, headers, etc. This behavior is controlled by RequestValidationMode property.
Now to override this behavior for CRM 2011, all you need to do is set <httpRuntime requestValidationMode="2.0" /> in your web.config. This setting will enforce request validation in .Net 2.0 mode i.e. only Form's input fields are checked for invalid HTML input.

FYI the Update Rollup 12 for Microsoft Dynamics CRM 4.0 is available for download at : http://support.microsoft.com/kb/2028381.

Andriy Butenko demonstrates a code sample to display number of Records in current view in CRM 4.0 using a Plugin on CRM Team Blog. Click here for the code and article.