This is a nasty bug that many others including myself have experienced...See the following discussions...
It has to do with the script step New Window or Go To Related Record in a New Window. It may have something to do with drawing the window offscreen...and seems to only affect Mac OS users.
If you can provide a file that consistently exhibits the issue...FileMaker would be interested in using it to test and develop a solution.
Hit return a couple of times and see if it will stop.