Ok, this part is simple but my guess is that 60% of travelers will do this:
When you buy a ticket for the train, even if it is a specific train and seat*, you must always validate the ticket in the yellow box at the end of each platform.
If you don’t, the penalty can be around 40 euro for each person! Considering that the Flornce to Pisa to ticket is only 5.70 euro, this is a big penalty.
We watched 5 other groups in our traincar get a lacture from the conductor (he was very nice and didn’t charge the fee) so this is a common thing to forget.
You have been warned!
*- officially this statement is not correct but you need to validate a ticket with a reserved seat of you take a different train. To be safe, validate everything.