Oh, you are very unfortunate!

If you were so close behind them that you rear-ended them that is your fault. You should always have enough distance to come to an emergency stop.

Brake check. Assuming you were tailing at a safe distance, you'll need to get yours looked at.

The appropriate response to seeing someone’s gas cap off is:

1) Shake your head
2) Cluck your tongue
3) Say, “That’s a shame.”

There’s a warning light in case there’s a problem. You don’t need to be a hero.

YDI for driving so close to them that you couldn't stop. Always leave enough room in front of you that you're able to just take your foot off the gas when the person in front of you brakes-- it saves your brakes from wear as well.

Not sure how well this would be received in America, but here in Japan, my gas cap was open and someone noticed. They got out of their car which was stopped directly behind me at a stop light, closed it, tapped my window to let me know what happened, and that was it

Wow. I didn't know SUV's could put their own gas gap back on!

For everyone saying it's the drivers fault for driving so close sure that's the general rule but the other driver intentionally slamming on their breaks is considered negligent driving and makes them at fault not OP.