Can You Guess What It Is?

The 4 pictures over here might give a bit of a clue.
I decided not to bore you with more ice shots.

Bore the same sized hole (using the same drill) in two bits of wood (actually the same bit of wood cut in two).
Glue a cylinder of wood in one of the holes (which is tight enough to require a mallet to get it in).
You get a really tight fit - right?
Wrong!
Well, it was ...... for about 2 minutes.
How the hell does that happen?