It is made clear that Sergei does not understand English, hence Phoebe's need for a translator. However, in the restaurant, when Monica says "The man's dog just died!", Sergei responds as in a tone that sounds like "big deal", as if he understood what Monica said.
In the heat of her argument with Ross, Rachel defends the long hours she has been putting in at her job by saying "do you know that this is the first time in my life I'm doing something I'm actually good at?!" Oddly enough, almost all of Rachel's work situations we had seen up to that point could be described as a disaster. Case in point: "I think I just shipped 3,000 bras to personnel!"
When Ross accidentally lights the flowers on fire at Rachel's office, the candle is not touching or close enough to the flowers and the flame starts in the center of the flowers away from the candle's flame.