On 29 November 2021, the crew of a Bombardier CRJ900 (C-GJZV) which had just taken off from San Diego at night were presented with a fuel imbalance indication. The imbalance slowly increased until, once level at FL340, a further annunciation indicated that the maximum permitted imbalance had been reached. Actioning the corresponding checklist did not resolve the problem, so procedurally recommended shutdown of the low fuel side engine was performed, followed by a MAYDAY diversion to Los Angeles. The imbalance was attributed to inadvertent crew selection of ‘gravity crossflow’ prior to takeoff instead of the immediately adjacent ‘crossflow auto override.'