As a tank myself - I have lost my nerve - during wrath when everyone was geared and I had just ding'd. The abuse I got trying to hold aggro with my greens vs their full epiced out dps with all CD's and the gogogogogogo mentality. Yep I lost my nerve. It took our guild tanks (yep both of em) to persuade me to go back - with both of them as dps, our raid healer as healing and another guildie as the last dps.
We went through the training run, then tried 3 randoms. The price of the 'gentle' intro was to get geared for a bit then they would ask a 'favour' I couldnt refuse.
A little while later (with wrath gear so easy to grind), my mostly kitted pally tank got the 'call'. I nearly s*at myself as they explained, the off tank was on holiday - and I was now the new Offtank (for the alts run - my main was dps in the main run). To my surprise I found that ICC was tbh easier than most heroics for me then. Mojo back, I started tanking on some pugs - and found with that bit of gear meant I COULD hold aggro vs Mr gogogogo (although I didnt just gogogogo- I waited for mana, CD's etc rather than let them push me - including letting him pull, die, and then taunting off the healer).
This became a regular thing - and I ended up MTing and RL the alt run a few times. It was wierd cos even though as the alt run we were significantly less experienced and geared than the main on - we equaled them on a number of occasions - boss for boss.
Anyhow in Cata I sort of lost interest in it after gearing my main, and then leveled a DK up (as dps while collecting tank gear), and just really couldnt be bothered to lvl my tank/healer pally.
After a while the old MT for the alt (who had changed guilds then - becoming the GL of the other guild) asked me for a hand - our MT came as dps and we pugged the rest. I warned everyone I was new to cata tanking - in my 80 pally, and icc gear.
We got stonecore. Considering the dps lock was heroic capable, I seriously feared for aggro, however we only had one death, and at the end, the healer asked if we could go again, and again. After running nothing but stonecore quite a few times, he is now Lvl 83 and still cannot face the quest chain (and no no tank stuff ever dropped!!!)