SFO last night fail. They asked for preboards then announced they'll board by group numbers. Dozens of people step forward. They then ask for group 0, and 1. When I got to the front I did ask why they can't get group 0 boarding right - the lady said that they should have a 1 minute delay before asking for group 1 but didn't know why that didn't happen. Both times I've flown from SFO recently have been a fail.