Recently at a competition, our drivebase (4 motor torque tank direct drive) was working fine for the first 8 qualification matches, and all of the practice matches beforehand. However, when we entered the elimination matches, one side of the motors would lose power. They hadn't stalled out, because they still managed to go at a relatively slow speed, but they didn't give us any pushing power while the other side was completely fine.
No idea why it would happen, and it would also show up in the autonomous: Our robot had a tendency to head towards the right (the side with the slowing motors). The motors still drove forwards, but didn't necessarily do so at full speed. Our robot is very light (all aluminum, just the mogo carrier).