Both of these happened in Dallas TX of all places. Not exactly known for celebrity sightings, apart from (NFL) Cowboys all over the place.
A million years ago my mum worked across the road from the only posh shopping centre in the city, and one day she was in there over lunch for who knows what, in a hurry and digging through her handbag, and she ran smack into only Elton John - really, she body checked him. Mum stood gobsmacked clutching her handbag, right in their path, as his security detail tried to move her out of the way. Sir Elton had mercy and merely smiled and said something like "excuse me dear". Her work wrote up the tale and kept it on their "Wall of Shame" forever. Mum didn't even apologise to him!
This one's my own and it's the only one I've got, but it's a doozy: I went to a film festival special showing of Hot Fuzz (only about half a million years ago). It had been out for a while, and that and Shaun were some of my favourite films, so my local mates treated me. We'd arrived far too early, so I, being the only smoker in the group, nipped out for a fag thinking I had plenty of time. I looked at the time and realised I didn't have near enough after all, so I ran back inside. Surreally, I looked to my left as I ran, and Dolph Lundgren was just popping up the escalator in a tuxedo, with several blokes trailing him. No time for faffing about trying to get past his security detail, so I ran for an elevator instead.
Just as the elevator doors close, a hand reaches in to pull them open again, and into the elevator walk none other than Simon Pegg, Nick Frost, and Edgar bloody Wright - just the three of them and me. I tried desperately not to annoy them and go completely fangirl but I wasn't able to stop myself from speaking to them, so I said something mortifying like "Oh hey! Sorry about the awful weather, it's usually much nicer here." They didn't seem to clock I had English accent (never remarked anyway), only Simon Pegg just said they'd had no time to notice, being ushered from bus to venue non stop. I said something like "aw that sounds awful! Well there are some surprisingly good places to drink round here and even a couple of proper expat pubs nearby if you've any time in town." Nick Frost rubbed his face and said thanks but they'd done it up too big in the last city and only wanted sleep after this show. They were friendly and gracious as can be on a short elevator ride with a stranger yapping at you.
I wished them luck and fun as the elevator doors opened. Then I essentially followed them through into the theatre (we all came in through a side entrance, surprising the crowd), lapping up the applause meant for them! None of my mates would have ever believed me had I not walked in effectively with them, and they all spent the first half of the show just staring at me, as if I had a proper explanation.