I don't know how he managed it, but my fairy wrasse managed to jump out and get
ontop 
of the eggcrate covers last week. At most there was a gap of about 3/8" between the two pieces. He must have hit it just right. He won't do it again though
by the time I found him he was dried out. I'm a bit more carefull about pushing the pieces to gether now to close the gap. Normally eggcrate is a pretty good method of keeping jumpers in the tank without restricting the air exchange and evaporation.