WOW!  I can't imagine having to cut up a plane that I had built because of
some detail like that.  While I use peel ply all the time in many of my other
composite projects, if you never knew what to look for, I can see someone
just not knowing that it was there.  There is a solution.  You can get peel
ply with indicator threads in it, I have both with the indicator threards
(woven red and black threads in with the white) and plain.  I was even
wondering about getting some printed that said "REMOVE BEFORE BONDING" but
that may be difficult since it is designed not to have anything stick to it.  
And if the printing transfered to the adherend, it might cause undue panic!

Dan Newland