Weird sprite appearances

 As my students are working (all brand new to Scratch) sometimes a random sprite is appearing on their stage for which there is no actual sprite in the program.  In other words, it does not appear in the list of sprites, but it does appear on the stage.  Any idea why this is happening or how to fix it, delete the stray sprite, etc?  They, of course, have no idea how it got there, so I can't figure out how to get rid of it.

Thanks, Jennifer