I'm not really sure if you mean accidentally tickled or if you mean the tickler intended to even though it wasn't part of the scene. In the former case, I'm sure it happens in rare, brief instances, though I can't recall any that I've personally seen (except when I'm tying my wife, usually with the intention of tickling her anyway, so her mind is already primed for it). In the latter, I know I've seen it a few times, but it's been a while and I can't recall specific instances. In those situations, the Dom/top either has a very close relationship with the sub/bottom, or tickling is included in the disclosure and release of liability terms.