anyone else got this error message

An error occured while loading the document. Fix the error and try loading the document again The error message follows:

Index was out of range, nmust be non-negative and less than the size of the collection. Parameter name index.

I'd built a product form and was happily inheriting from that then this message started to appear as soon as I tried to access the inherited form.

I've even tried a new one and creating an ordinary form and using the inherits keyword.

I suspect I will have to rebuild the product form from scratch but if anyone else has expereinced this and knows a way around it please let me know.

thanks