Invalid Answer pop Up

Hi everyone,

                         I have a a random 'invalid answer' pop that is appearing in a few of my question slides (see attached image) and can't figure out how it is being generated and more importantly how to stop it. 

The question has a 'correct' and 'incorrect' layer.  If you enter the correct answer the correct layer appears however if you put an incorrect answer in the 'invalid answer pop up appears.

Any ideas gratefully received.

Thanks

Shannon