In my case it's mostly happened when practitioners do that very assertive feeling-around of your abdominal area; invariably I twitch and flinch and they apologize and say "Did that hurt" and I have to say it tickled. Once the woman who was performing the examination nodded and said "I know how that is, believe me" and proceeded to be extremely careful about trying not to tickle me again.
But once I went to the ER (for a scare that turned out to be nothing) and they had me in one of those curtained-off rooms and the middle-aged nurse grabbed my ankle and -- just before she ran a blunt instrument firmly up each of my soles, one maneuver on a checklist of maneuvers she was performing -- said something like "Wait, you're ticklish, aren't you?" She either said that or "Wait, I bet you're ticklish." Then she performed the maneuver and my whole body jolted as barks of laughter came bursting forth, and then she moved on and was gone.
My response was "That's weird, she just thought I looked like someone who was ticklish?" It didn't occur to me until much later that maybe a previous practitioner had put some kind of a note in my record. That strikes me as unlikely, especially since this visit was in a different city from where I usually receive medical care, but either way -- my ticklish response to the test apparently was fully anticipated.