It’s a thorn in the foot

This was uttered at a presentation where the speaker was describing a problem.  It is a mash up of “thorn in the side” and possibly “shoot yourself in the foot”, but probably the speaker was thinking of the Aesop fable where the lion has a thorn in his paw.  Or, as “my ol’ pal” notes, we sometimes get slivers in our feet when walking barefoot.  Thanks to Cecily for sending this one to me!