Revealing mistake: Just before we see everyone setting the wedding up outside, there is a sweep shot of the city with snow falling. It's very obvious that this snow is added digitally and it's very badly done. There is no snow on any of the buildings, and there is no snow hitting the surface of the water.
Revealing mistake: At the wedding, none of the snow melts when it has been sitting on people for several minutes. Occasionally when it gets brushed off clothes it just sticks to hands and sleeves, again for several minutes without melting.
Continuity mistake: After Phoebe tells Monica that she is fired from being her wedding planner, Monica says that she has never been fired. However, in "The One with Five Steaks and an Eggplant" Monica gets fired for taking the kickbacks.