I’ve run into this error before and last night it reared its ugly head again. You’ll find it from time to time when switching to Design View in Visual Studio to work with custom server controls. The bug is in Service Pack 1 of VS and can be quite annoying.
It has been submitted to Microsoft and they claim to be working on a solution which will “fix it in subsequent releases” of Visual Studio. Unfortunately, developers (like me) don’t want their productivity to suffer while we all wait for a fix from Microsoft. So here are a couple of steps you can take to try to resolve this issue yourself:
1. Close and restart Visual Studio. Some developers claim this solved the problem for them. I cannot confirm that this works because it did not solve the problem for me, but it’s certainly worth a try since it’s quick and does not require you to modify your system in any way.
2. Delete all project assemblies from your ProjectAssemblies directory.
The second solution did the trick for me. Hopefully one of the above solutions worked for you. If you have run into this bug and found an alternate solution, please let me know and I’ll make sure it gets added to the list.