Only two times really come to mind for me. During IFR trainign (In VMC) ATC told us we had traffic 1 o'clock, i'm under the hood, and my instructor seems to be "dilly dalling" so he didn't look up.Little later, ATC says, traffic is a half mile, and they'd highly recommend a change in altitude, at that point i looked over, saw him, and pushed the stick forward, then watched the red white and blue v-tail bonanza go right over. about that time, my instructor said , "hmmm, i think that would have been a collision."

And when i was instructing a future flight instructor, we were @ 6,000 doing some maneuvers, and he was having a hard time keeping his altitude. Because he was being an instructor, i decided not to correct him, and let him make the mistake and tell me what was happening, etc. Well, we ended up about 100 ft low, and then we noticed a twin go overhead, kinda freaked both of us out. I told him to climb back to six, and when we did, the other aircraft was level with us, kinda freaky to think all i would have had to do was say "get your altitude fixed" and boom, we mighta been tumbling towards the earth in a bent piece of tin.