As with so many of you, this is one of those that number too many to count...
...I've passed up
SOOOOOOOOOO many opportunities previously in life, back when this was my big, deep, dark secret that I was ashamed of. I didn't want to 'out' myself by taking advantage of perfect chances.
The first one that occurs to me, reading this thread, though, was my first crush. I had it BAD for this girl, and I still wonder about her to this day. Her name was Julie, and she was spellbinding. They say you fall hardest the first time you fall, and so it was with Julie. I fell in love with her in 6th grade, and it lasted until my sophomore year of high school, when I moved away. Never told her how I felt, afraid of rejection. I played the "Mysterious Admirer" game, though, even though everybody knew it was me. Kids are so stupid (and cruel). But I digress.
There were actually TWO instances where I passed up opportunities to tickle her. I can't recall the chronological order they happened in, but that doesn't really matter, does it. One of them was during summer vacation from school, and the Parks & Recreation Center has these trips you could sign up for for next to nothing, to water slide parks, tours, etc... We were on the same trip together, and I was simultaneously in heaven and hell. There were several kids from our class on the trip too, so we all had reason to hang out together. We ducked out of sight under this bridge over a creek, where the chaperones couldn't see us, and just chilled out, talking about the stuff we weren't supposed to, using the language we weren't supposed to, you know how kids are. Julie was always pretty athletic and adventurous, and she jumped up and grabbed a pipe on the bottom of the bridge, out over the creek, and hung there for a little bit before deciding to try to swing over to the other side.

She was wearing a white tube-top.
😱 So all stretched out with her arms up over her head, I saw an upperbody tickler's dream country displayed... smooth underarms and belly, just begging to be tickled as she hung there. Had it been just the two of us, I may have overcome my bashfulness and actually tried to tickle her as a way of finally breaking the ice, but with the others around, I held my ground, not wanting witnesses to something that was this intimate to me. At this point, however, the REAL problem begins. As she's swinging there, her tube-top begins to slip down.

Oh no! This girl I love is about to be embarrassed in front of everybody. She wasn't particularly well-endowed, but I didn't care... this was potentially humiliating for her, and I wanted to rescue her. As I'm scanning the rocks I'm going to try to jump across to go and help her down as gracefully as possible, it slips further down and she gives up her grip to save herself the embarassment, and goes into the creek. It wasn't that deep, so she didn't fall or anything, but I'd missed a couple chances that day that I'd kick myself for years to come for.
Then there was another time, when for Phys. Ed class, both the Boys'
AND the Girls' classes (this was unprecedented) got to go to the city pool and swim for P.E. And there she was, in a bikini, making my heart do somersaults. I spent the period doing ridiculous antics with my buddies, as boys will do, but all the while keeping an eye on her and sighing on the inside. Then, Mr. K (the coach/teacher in charge of the P.E. class) told us it was time to head back, so everyone got outta the pool and grabbed towels and shoes and started heading back. She and I were the last two in the pool, and she swims past me gracefully. I
HAD to do
SOMETHING... I was aching for her. She knew, everybody knew, it was this whole idiotic thing, but neither of us ever talked about it. Maybe she was as afraid as I was... I'll never know. But once again, I digress. As she's stroking past me, I can't resist the need to act on her proximity, so I reach out and put my hand on the back of her head and dunk her a little bit, then turned and "ran" away through the water (as much as one can "run" in a pool), although truth be told, I wasn't really trying to get away, just giving the appearance of having been a brat and trying to escape. She caught me easily from behind, and grabbed me, wrapped her arms around my neck and shoulders, and her legs around my waist, riding me piggy-back and sending me to the moon at her skin touching mine. Her feet were right there in front of me, crossed to lock her legs around me, so I could've easily tickled her to get her off me. Not that I wanted her off me, mind you. I was in heaven. What I really wanted was to spin to face her and kiss her, but not only would that have "given me away" and ended the ridiculous facade of my Mysterious Admirer game, but we still had an audience in the form of the P.E. teachers, so rather than tickle her like I could've to dislodge her, I floundered over to the deeper part of the middle and let us sink, so she'd have to let go to get air.
*HEAVY SIGH* Did I mention how stupid kids are? Ennyhoo, them's my two biggest missed tickles. Amongst other things.