It depends on my mood. I've had people cut the queue to get on the bus first. Normally I just let it go but I've had the odd person almost push me to get in front - they may get a comment passed loudly to make sure they hear. I once had a woman who basically ran to get in front of me and my trolley at the supermarket checkout queue & she "mistakenly" said - "you don't mind do you?" She annoyed me so much I said "actually yes I do". She didn't expect that response hahahaha. People don't expect to be picked up on their rude behaviour & it may make them think twice before they do it again.
I did this week-end and my son and I were told to 'get stuffed' by an old man. He was shocked at the mans rudeness, I was just, again, unsurprised and slightly disappointed. I usually say something. I know I have accidentally joined a line not realising I had cut in and been very apologetic when it was pointed out to me.