(I am just seeing this thread).
10/06/2016, UA 1699, IAH-SFO, B737-900, 6:25pm departure.
During the first take-off, the pilot slammed on the brakes. It sounded like we busted a tire, but apparently we did not. The pilot said that a cargo latch was not properly latched, and that we had to return to the gate.
After a while we were cleared for take-off again. During the second take-off, and just before going airborne, the pilot again slammed on the brakes. The pilot said that the cargo latch was still not properly latched?!
We had to wait for a gate to open. As the pax deplaned, there was chaos at the gate - lots of chaos. There was ONE UA gate agent to assist the pax.
To make a long story short, I called UA and got on the next departing IAH-SFO. UA (on the phone) did not want to offload me and book me on the next flight. They eventually did after I escalated the issue.