I'm debugging a call to a Stored Procedure in SSMS (2012). Frequently, and various random points in the code, a dialog pops up:
Microsoft SQL Server Management Studio: Unable to Step. Invalid Operation.
Once I dismiss the dialog, it appears that debugging CAN continue... I can continue to step through the code to the end, without any apparent problems. So far....
I had the same problem. After some searching I found this page which suggest that this caused by a race condition. Although the workaround given there wasn't applicable for me, after stopping every other debugging process (Visual Studio) these annoying error messages didn't show up anymore.