Thanks for all the answers. Since a few people have asked, this was UA SFO-LAX on a 737.

What bothers me is that the FA told me to just go out and ask to get put on the next flight, as if this was a common occurrence. When I went out, the GA seemed busy and didn't want to deal with me, so she passed me on to someone else.

Here is the airline's carry-on baggage page: https://www.united.com/CMS/en-US/tra...eCarry-On.aspx It clearly says that "each traveler can bring on board one carry-on bag plus one personal item free of charge". There are no asterisks or disclaimers anywhere about room not being available. Perhaps it's in the fine print somewhere else.

I don't think it's unreasonable to expect that airlines will provide the service they advertise. Of course sometimes it's just not possible, but in those cases they should work with you to provide another alternative rather than saying sorry, you're out of luck.