I've had a few times where I thought an adult was "possibly" or "probably" wearing a diaper, but those don't count IMO. Only once have I seen a definite diaper sighting.... and it was most excellent.
It was at a Phish concert in 2012, there was a woman who looked to be about 25ish, short, petite, brunette.... very attractive.... she was wearing a tank top and one of those long "hippie" skirts. She had on some off brand goodnites, I've seen them online, but don't know the specific name of them or where they sell them at.... they had peace signs in different colors as a pattern, very appropriate for the concert lol. They were sticking out the top of her skirt about 3 inches in the back, and about an inch in the front. She was dancing with a guy who definitely would've noticed the diaper and he made no attempt to tell her to pull her shirt down so I kind of think it was probably intentional and ABDL related. I watched her dance for about 10 minutes out the corner of my eye before my friends were like "c'mon lets move closer to the stage".
Then not technically a diaper sighting.... but for 34 years I never saw a woman between the ages of 20-50 purchase adult diapers, then I saw it happen twice in one week at Wal-Mart!
Last Saturday a woman of about 32, looked to be either multi-racial or light-skinned African-American, average height, curvy figure was in the incontinence aisle with her boyfriend. She was comparing the Depend products, opened a package and looked at the design. I was in the toothpaste aisle adjacent to her trying not to stare... she said "damn it, they don't have the ones in this design in Medium!" I wasn't looking at them at this point, but heard her tell her boyfriend "distract my cousins, I'm going to run to the register and I don't want them to know." I saw the boyfriend with the cousins in the grocery aisle a few minutes later, didn't see her again and don't know what kind of pullups she bought.
Then yesterday I saw a tall white woman, average weight, looked to be about 38 come up next to me in the dog food aisle. I grabbed the dog food I get my dog a second later and was walking away while she was standing there browsing the different types of dog food. I noticed she had a pack of Always Discreet Underwear in her shopping cart....