When you are in a shop and it has reached closing time, the owner will tell you to leave before the game automatically kicks you out. Interestingly, when I stayed in Gracie Grace after hours, I was allowed to stay indefinitely. I was allowed to buy items just like normal. Then when I went down to the 2nd floor, Timmy promptly kicked me out.
I see no logical reason as to why one particular shopkeeper would let you stay like that. This leads me to believe it was an error by the developers. I find it weird, since Nintendo games are always so well polished. I guess something as small as this can occasionally slip by.
Haven't seen this as I'm generally not in the stores at closing time, its a small oversight tho, if it is one.
You are allowed to stay in Ables after hours - well I think so,as its important if designing a pattern in there - but again I've not been doing that at closing time.
What I used to be careful about is when I went to dream towns which had Wendell & patterns - post 10pm I wasn't going to be able to save them with Mable.
I've only been kicked out once at closing time, and haven't tried with other games since my GC days. There, I remember staying in the shop all night. Nook kept trying to get me to leave, and the music was different, but I wasn't forced out.
Perhaps they ran into people doing that too much, and since the shops keep specific hours, they have changed it to where the NBs can actually boot your butt out of the store.