Realising that sometimes customers just don't listen...or they don't even bother trying to understand what you are doing.

It's not the first time that a set of requirements have been agreed and then they turn round and go

Cust: "but we wanted it that way"
Us: "did you ask it for us? Cause it would have been in the statement of work..."
Cust: "no but sure, it won't take yous that long. So yous can still fit it in"
Us: fuuuu....

No matter how good things are going, a customer somewhere is going to fuck you over because "you should have known" about their requirements...