In the wedding dress scene with Monica, Rachel and Phoebe wearing dresses drinking beer, Monica has black tights on, which she didn't have on earlier - in coffee house, she has bare legs as Phoebe sees the garter - so she must have put on tights with the wedding dress. That just doesn't make sense!
The inconsistencies due to storylines from years apart not matching up properly are less annoying that blatant continuity errors.
Also, when it was show first, we didn't have widescreen TV, but now in re-runs you can see more of the "edges"
EG when Chandler wakes up with Janice (the one where Joey got with her friend, and they didn't realise Janice was coming), in the original broadcast you couldn't see Janice in bed until it panned out a bit. Now on reruns you can see her there.
And in the one where they think the stripper stole Ross's wedding ring (for Emily), they book the stripper to come to Chandler's office. You can now see Joey, slightly off camera at the edge, laughing into his hand (he isn't meant to be on screen, so I assume he was trying to hold it together instead of ruining the shot)
Now that I mention Ross's ring for Emily- that his Grandmother smuggled into America or something - would that not have gone to Carol? OK, she might have given it back, but would you really recycle it? Then when Rachel has Emma, his mum gives him ANOTHER ring (or the same one again??) to propose.
My last one (for now, I need to get a hobby!) - it drives me nuts how "old" the Gellers and Richard act, when they are meant to be about 50! Richard quips that it is fun hanging out with Chandler and Joey because all his own friends talk about is guess who died shovelling snow.