Rachel is seen coming into the coffee shop with the gang from the softball game that they won, clearly having played; however, in "The One with the Football," Rachel is picked last when they divide up the teams and is an obvious sports liability.
Joey is present for the conversation regarding how parents claim pets go to live on a farm when they die. However, in "The Last One" he tells Phoebe that Chandler took the chick and the duck to a farm, but he isn't aware that this meant they died because he comments on how it's a shame that people can't visit.
Yellow coffee cup jumps from Rachel's hands to Phoebe's hands instantly during camera angle switch at Central Perk.
Phoebe gives Chandler $7000 if he never smokes again. Given the fact that Chandler smokes on and off again throughout the rest of the series, he should give Phoebe her money back; yet there's no mention of the money in the rest of the series.
When Monica brings Joey in to tell the gang she's going to break up with Alan, Joey is in his bathrobe and mentions it would have been nice to be able to put on some underwear. It is established later in the series that Joey doesn't wear underwear (with one exception: "You don't go commando in another man's fatigues!")
Inexplicably, the entire gang are on a softball team except for Monica who later we find out, is fiercely competitive so it would stand to reason that she would definitely be playing on this team as well.
Phoebe reads a letter from her bank out loud and when she reads her own last name "Buffay" she pronounces it differently than she does the rest of the series. She pronounces it more like a food "buffet".
While it is still very early in the series and character traits haven't been all that clearly defined yet, Phoebe (Lisa Kudrow) reacts to glibly about finding a severed thumb in a soda can, that is, she is only concerned about the universe spoiling her day and eventually receiving undeserved compensation. The Phoebe soon to emerge would have been simply devastated about somebody's injury.