6 down!
On the review thing I don't believe you always know when you've hit it. If you're swinging hard at a ball you may not feel a really thin edge and the same applies if you hit your pad/boot/the ground at the same time as you nick the ball. Mikey's example seems a bit extreme though.
The talk of edges reminds me of an incident in the second match I played for my current team. I was bowling and the opposition opener edged one behind. You could clearly see the path of the ball change so I was more celebrating than appealing. The batsman just stood there. I turned and looked at the umpire (the opposition captain) who appeared unmoved. "WHAT?" I half-shouted. "I'm giving him the opportunity to walk" said the umpire, as our captain ran up from mid off and got in the his face. Hands on hips, I turned and looked at the batsman, who stayed exactly where he was. I looked back at the umpire who after a short pause loudly called down the pitch "Mike!* I said I'M GIVING YOU THE CHANCE TO WALK!" After a slightly longer pause, off went the batsman, who'd technically walked…
* I picked the name Mike out of the air, I can't remember the actual name