A CAN node keeps going "bus-off." What does that mean and what causes it?

Bus-off is the most severe fault-confinement state: a node's transmit error counter has exceeded the threshold (255), so the CAN controller disconnects itself from the bus entirely, it stops transmitting and receiving, to protect the network from a node that's generating too many errors. It's reached after sustained transmit errors, so the causes are usually physical or configuration-level rather than a one-off glitch: wrong or missing termination causing reflections and bit errors; a bit-timing/sample-point or bit-rate mismatch between this node and the rest of the bus (so its bits never line up, every frame errors); wiring faults (CAN_H/CAN_L shorted, swapped, or open); being the only active node so no one ACKs its frames (an unACKed frame is a transmit error, and a lone node retransmitting forever will climb to bus-off); or a transceiver/hardware fault. Recovery requires a defined sequence, the controller must observe a number of idle bit sequences (128 occurrences of 11 recessive bits) before rejoining, and firmware typically must explicitly restart the controller and may implement a back-off/retry policy. So the debugging path is: check termination (~60 Ω), confirm bit rate and sample point match all nodes, verify wiring and that at least one other node is present to ACK, and inspect the transceiver. Bus-off is a symptom of a real bus problem, not something to just auto-restart past.