Ha, yup. I posted earlier but I had to wear diapers all throughout my childhood so naturally, I got caught "playing with my tinkle," as my mom called it. So I started putting toys down there instead to "innocently" cause some friction. My action figures at first but I didn't want them to get wet so I switched to random objects like Crayons, pens, bits of plastic, Legos, etc.
This was also aided and abetted by a couple times when I was sitting there in just a diaper, eating crackers or cookies, and crumbs got down in my diaper -- plus a couple times when my grandmother made me a makeshift diaper out of a towel and a bread bag but didn't shake out the bread crumbs first. Then one morning at breakfast when I was around 6 or 7 and everyone else left the room, I got an idea and poured my bowl of Frosted Flakes down the front of my Pamper.
Naturally, my mom saw when she went to change me but I thought fast, remembered her saying not to dump milk in the trash, and said I did it to keep from making a mess. Not sure if she believed me but I think she thought it was funny so I got away with it once or twice more.
I also have dim memories of someone dumping food or a wrapper or something in my diaper as a joke when I was younger (4ish) but that's pretty cloudy. Plus memories of some kind of kids' book that had a drawing of a mean brother putting a mouse down his sister's dress (but imagining he put it in her diaper instead), and another memory from around 5 of hearing about a cousin pouring her oatmeal down her diaper, but mishearing it and thinking my dad did it as a joke, wondering if they'd do that to me next, etc.
Anyway, all this culminated in a recurring WAM-related fantasy when I was 12 or so. In the fantasy, I had to wear nothing but a diaper to a restaurant or food court, and everybody would scrape their plates into my diaper (instead of the trash can) when they were done. Around that time, I started stealing diapers from aunts' diaper bags, but I was still too shy/ashamed to masturbate or use them, so I'd put toys down their instead, walk around, almost instantly orgasm, then wonder why I was so weird for not masturbating like "regular" boys.
Brains are weird, yo.