I made a custom search web part for SharePoint as described at: http://msdn2.microsoft.com/en-us/library/ms551453.aspx

While developing, in number of occasions, I received an error: „Cannot import this web part”. The error seemed to appear randomly.
As usual, the solution was simple. My SharePoint portal was extended to two sites.

Simple rule:

• If you are deploying web parts to SharePoint portals you need to deploy to each \app_bin folder

Recommendation:

• While you are developing web parts it is recommended that you deploy them to sites that are not extended. If you have an extended portal you will encounter numerous problems like the one mentioned above. Also, every time you rebuild your DLL you will need to remove all web parts from SharePoint pages and add them again.