Oh yea, definitely had quite a few. Its one of the joys of tickling people often- you're gonna get an equal if not greater number of negative responses than positive. One girl left herself perfectly open- laying there on her back, hands way over her head, shirt lifting up just a bit... I had to go in for the kill... and got nothing. She just looked at me, said "sorry, not ticklish". "Not... not at all?" "nope." "anywhere? 0_o -tries under arms-" "nada."
However, tickling someone and having them be not ticklish can sometimes be MUCH better than tickling someone who turns out to be hyper ticklish. Hyper ticklish is a tickler's worst nightmare. Tickled a girl one time who literally hit the ground trying to get away. In fact, I only started tickling her; her reaction was so severe my hands got caught in the fray while I tried to remove them (almost instantaneously) which only tickled her more. She ended up injuring herself and me in the process, and was royally PISSED. That didn't go well. lol.
There's a nice in between ticklishness that I love. Super ticklish to the point that they laugh loud enough to wake up neighbors, but not so ticklish that they cant stand even having you scratch their back without squirming away. 😛