Once a gift certificate from any business establishment is ripped, it is considered invalid.
Phoebe is upset about the gift certificate that Rachel received for a chain spa because they put independent massage therapists out of business. There should be no issue with Rachel using it. The purchase of the certificate was what she should've taken issue with.
Chandler mentions Monica's class trip to the Hershey Factory. Hershey Factory tours ended in 1973 when Chocolate World was opened, giving simulated factory tours on an automated ride. Monica wouldn't have been old enough to go to the factory on a class trip.
Monica tells Chandler that they have been trying for a baby for over a year. It can't have been anywhere near that long, since they started trying on the day Emma was born, and Emma's first birthday wasn't until the following season.
Chandler asks if Charlie is a girl by saying that he missed most of the party (The One With The Soap Opera Party), but he was present at the moment Ross first met her at Central Perk.