I remember when my lot played Leeds, you'd sold all your away ticket allocation, so some Leeds fans got in our section, I had one middle aged guy sitting next to me, he was no bother, then when you scored, I saw all his mates dotted about behind us lol. We didn't grass or nowt, but once the stewards eventually realised, it became obvious it was more sensible in not trying to remove them. Although some of my lot give me a headache and started moaning, I'm like just stfu (to my lot).
I do it a lot around the Midlands but know not to jump up, did it once at Walsall but all was good there was about 30 of us when I looked around.