Yesterday I was playing some ranked matches. I found that a couple of times I had the drop on someone with the .50 cal (support) and nailed them from fairly close range, and point-blank. I then watched in amazement as they turned around, came at me and killed me melee....

Happened more than once, and needless to say we lost!
I just figured that I was lagged out, and cursed my ISP!
Then later that day I'd just finished another ranked match, and someone on the losing side claimed that the lead scorer on our team was cheating, and using the 'invincibiliy' glitch.... I'd not heard of this before.
Anyone had any similar experience?