Very true. Sadly, those flights most likely to have a problem for me usually involve involve a partner airline and an international flight where online checkin may not be possible due to doc check requirements.

When I was flying LH/UA, Lufthansa would usually not let me check in for the feeder to FRA because they wanted to check my eligibility to enter the US. When I fly AS/DE to Europe, AS usually wants to see me at their desk before issuing me the boarding pass for their feeder flight. And last time they had a problem that then required a call to the help desk, so yes, arrive early at the checkin desk!