Especially if you fly open jaw, you may want to spend a couple hours scoping out the airport. Same applies to train stations.
Incident 1: flight out of Munich at 6:00am. S1 and S8 both run to the airport but feeder trains are less available at 4:00 in the morning so it is not a straight shot like later in the day would be. I spent a couple hours the day before leaving, rode the S8 out to the airport, found the gate in T1 and it was a piece of cake the next day not having to backtrack or rush.
Incident 2: Son and daughter-in-law were scheduled to leave Barcelona at 10:00. They got on the wrong train and headed somewhere into the Spanish countryside. Missed their flight(s) and had to buy 1-way tickets for the next day. Those tickets cost about as much as the whole rest of their trip.
Some things work out, but sometimes it pays to check details.
Bob: you make a good point and it was especially helpful for you to share the problem that your son and daughter in law had taking the wrong train to the airport and missing their flight and having to buy one-way tickets costing about as much as the whole rest of the trip. This doesn't much happen to us in the US because we just drive the rental car back to the airport, no problem and no train to figure out. Some of the biggest problems on European trips can be the problems that blindsided us, that we never saw coming, that we didn't know that we didn't know.