It happens more often than I would like to - being stuck with a non-tank kind of tank for an instance. You know, the ones without any tank gear whatsoever and who just look dumb when you ask them about crit immunity.
Some days ago me and a friend were doing UK normal on our lowbie alts. After waiting forever we got a group with a dk-tank. Since I am somewhat of a sceptic of dk-tanks I quickly inspected him and noticed that he had no tank-gear whatsoever. He had in fact the entire starter dk-set still on him. Nothing wrong with that, it is a great set, but it doesn't have any tank stats and is starting to wear out at around level 65. And he was level 69. We wiped on every trash pull and before we had killed the first boss we asked the tank to leave, since he obviously wasn't a tank. Don't get me wrong, playing alot of tanks have made me extremely patient with newbie tanks. But this hadn't shown any interest or insight into the possibility that there was more to tanking than being in Frost Presence. Luckily enough he didn't nerdrage when I kindly proposed him of getting the Cobalt Set. Just the day after that another friend of mine gets yet another non-geared dk tank, but this time for heroic toc. Newly dinged probably, but how is it possible that the notion of tanking has escaped him completely? What's with all these dk tanks?
Now UK happens to be a rather difficult instance for those levels. The mobs hit considerably harder than the correspondingly leveled instances of Outlands. Also the Outlands gear doesn't sufficiently prepare characters for what is coming in UK (unless you're raid geared), which is why there is a level 70 set for every class and spec. For tanks there is the Cobalt set, which I mentioned in my warrior guide. This set is of course good for any kind of tank. The problem is you have to be level 70 to dress it, but you can enter UK already at level 69. Imo this shouldn't be so, most people are definitely not geared enough at level 69 to complete UK. This isn't just an issue for tanks, but for any class and spec. But one notices lack of proper gear the fastest in a tank. This means as a tank it is the most important that your gear is up to the task, and even more unfortunate there are tanks that don't know about this at all.
Interestingly enough this only happens with death knights. Well actually it's not that strange at all, let's look at why people playing death knights thing that frost presence = tank mode.
One of the first question marks raised when Blizzard first announced they were going to make Death Knights start at level 55 was whether someone would actually be able to learn a class properly when playing it for so few levels. Playing a class from level 1 is the best way to learn everything about how they work, goes the reasoning. There is a point to that. Looking at the vast amount of level 80 player of other classes than Death Knights that don't seem to know heads from tails about their class argues against this point however. They can't all be ebayed! I still think it is part of the issue though.
A bigger problem is the fact that when leveling from level 1-80 most classes will at least at some point know what it feels like to suck. But death knights being a hero class and all, they have to be special. So Blizzard throws some awesome gear onto them, which makes their first levels easy as stealing candy from a child. Even when tanking. So the first three levels before going to Outlands, Dk kick ass at everything they do, and for the upcoming 8 levels before Northrend Dks continue to kick ass. As a dk you learn that it doesn't matter what you do, you'll probably kick ass.
Another issue is the fact that there is no difference between dps dks and tank dks in any pther aspect than what gear they have. Sure there are some talents that are more about tanking and some talents that are more about dpsing, but compared to paladin/warriors these differences are extremely small. This gives the false impression that frost presence is the only real difference between whether you're a tank or not as a dk. Frost presence = tank mode. Druids could have had this problem, but since they get crit immunity from a talent this is way less of a problem for them at these levels.
Most classes lol their way through Outlands. And then comes UK and makes everything difficult, even for DKs. Especially for Dks, since they're about the only ones who run in there without even a hint of tank gear and think they can tank. They're actually convinced they can, but they had no trouble whatsoever in doing so one level ago. It might've worked for Ramparts dude, but it won't work this time. You'll die miserably and blame the healer, who probably doesn't have good enough gear either.
So please Dks, learn about what it means to tank. Frost Presence won't be good enough after Outlands, trust me. You will have to get defense and preferably other tank stats like dodge/parry and stamina as well.