MFC uses the ASSERT macro that does exactly the same. An assertion error is always a bad thing, and you should make sure you don't have any. If you ignore assertion errors, then you may get some weird errors in totally unrelated places that are hard to track.

Click "retry" when the assertion message comes, then "debug" in the next dialog. You will be placed at the line where the assertion failed. Using the call stack, you can then find out which function of yours called the function that asserted. You shold then set a breakpoint there and find out where the error comes from.