I’ve been getting this error every now and then.

Usually this problem is caused by having 2 controls with the same name.  It can be that they are in different folders as well.

I came across this post today:

http://personalinertia.blogspot.com/2007/06/there-bug-in-compiler.html

Well, a quick search online told me that this might be a result of a known bug in the compiler. The fix was easy, you have to compile your app in non-batch -mode. How do you do this, I hear you asking. Simple: enter the compilation section of your web.config, and set batch=”false”, as so:

<compilation debug=”true” batch=”false”>
…
</compilation>