I've been confused for my own mother before (buying something with a credit card with my deadname on it, having a doctor's office receptionist double-check that I am the patient and not calling on his behalf because I "sounded more mature" on the phone and parents can't make appointments for adults), but I think the funniest wrong conclusion I've had strangers make about trans things I've had around was this time I was putting my old shit up at a flea market. I was selling stuff at a "this shit's gotta go before I'm about to move and I'm not hauling these back to another city"-price, and this old man who apparently frequented there to find stuff that's actually valuable being sold at such prices came to have a look at the goods while I was setting them up.
He was interested in some of my dishware, and asked politely if he could have a look at them while I was unpacking, and chatting with me while he waited for me to be done since he couldn't buy them before I was done (I let him have dibs, but the cash register wouldn't ring them up before I was all set). He apparently noticed that some of my silverware that I'd gotten as graduation gifts and literally never used had my old name engraved on them, but he didn't say anything about that.
But when I put up my old graduation dress - which wasn't worth much, cheap fake silk but it was a nice cut - he remarked "she must've left in a hurry to leave that behind." Since the finnish language doesn't have gendered pronouns, I didn't immediately make the connection that he was referring to someone female before he gestured at the dress. Once it clicked, I figured the best course of action was to just nod that yeah, she's not coming back for those and I'm not keeping them.
Dude had concluded that my deadname must be an ex that dumped my ass so fucking fast she didn't even pack her silverware or nice clothes.