
Of Warcraft... nah that joke is getting old.
However, some tanks in random groups make it out like it is.
Ok, so in raids we generally either
mark up the kill order like Star, Orange, Pink, Green, etc, etc. At other times there's too much messing around and only a single target to kill anyhow, so we mark up the tanks instead. Easier for the healers to see where the important players are, and easier for the melee to see what's the front or back side of a mob.
Since I'm so used to the tank having a mark for trash, I prefer to mark up my focus target in 5 man random groups as well. This seems to lead to plenty of funny situations. Seriously, not just funny, but absolutely hilarious how some kids get their pants in such a knot that they fake disconnect, or start pointing a finger at the poor healer for "targeting" them.
Here's a situation:
I join the random 5 man heroic dungeon queue thing for the daily frost badges. A group is found, and we're teleported to Violet Hold. I look for the player with the most health and check whether he's using some defence gear to determine whether he's indeed the tank. Some times it's rather obvious, some times it's not. Then, I do "/focus". Now I have the tank on focus target so my tricks of the trade macro can do it's job. Feel free to copy it, it's nice.
#showtooltip Tricks of the Trade
/cast [target=focus, exists] Tricks of the Trade; Tricks of the trade
It basically shows the tricks of the trade tooltip, and casts tricks of the trade on your target if you have a focus, otherwise it just casts tricks of the trade and you can use it as you normally would.
Next I mark the tank up with a green triangle. Not the old fashioned way, no way, using a macro that also uses the focus target. Again, feel free to copy it and annoy tanks with it later.
/script SetRaidTargetIcon("focus", 4);
Next, one of a few things may happen.
#1: The tank ignores the mark and the run proceeds smooth and fast.
#2: The tank comments about the mark, asking what it's for but does not remove it. I'll tell him it helps us see where he's running off since random tanks all look different. Also, that it helps the melee with positioning. Thus ensuring more dps, and a higher survivability rating for himself as the healer can find him more easily as well. The run takes a few seconds longer to get started, but after that it's generally a smooth ride.
#3: The tank removes the mark, after which I re-apply it, after which he removes it again, after which I re-apply it, after which he gets frustrated and starts cussing about it in party chat and removes it, after which I re-apply it, etc. Eventually the tank will either randomly start pointing fingers or simply quits the group.
The first two situations are generally fine. After all it's not the end of the world to have to run through a 20 minute instance with a mark above you. Right? Funny enough, it seems situation three occurs more often than you'd think.
Gundrak

I made a screenshot. I'm quite sure of it, but sadly I can't find it again right now. So, you'll just have to take my word for it. We went in, and it was a classic situation-3. It was a plate user, quite a few weeks ago now, so the memory of the exact event is slipping somewhat. We had a druid healer, poor guy.
Anyhow, we went in and I did my usual thing, focus, mark, start dps.
Sure enough the tank didn't agree to my methods, but I wasn't about to tell him that I was the one that was marking him up. It took about ten minutes of him threatening to leave if the marking continued (I do SO love it when they claim that, after all we can just get another tank in his stead, and he gets no marks and can't sign up for the next 15 minutes), obviously I keeps marking him. My favorite disclaimer is always "I'm not even targetting you", which I obviously don't need to since I have a focus target, but I wasn't about to tell him that EITHER.
After a while he started getting really paranoid, looking for whoever
was targeting him. He noticed the healer (Druid sorry, seriously, but it was fun to see him squirm so much) was targeting him pretty often. Well duh, he's HEALING YOU you mongul. And he proceeded to threaten to kick the druid if he kept marking him. After which I happily continued doing exactly that.
After the giant dino boss the poor druid got frustrated with the tank, (I think it was a death knight actually) and switched to bear form and refused to heal any longer. Death knights being overpowered don't need that much healing anyhow, so we continued on to the last few packs while the druid lagged behind calling the tank names and the tank calling the druid names. Right after the third before last pack the tank vote kicked the druid. (I did vote no, but alas, he still got kicked) Some priest replaced him, and we killed the last boss. I'd have re-marked the tank after the priest came in, but that'd mean I wasted 30 minutes to get the two frost badges, can't have that :P
Pit of Saron
Not so much a Situation-3, but I still felt like naming and shaming these two idiots. Comedy either way. A tank and a dps left after the first boss in the Pit of Saron (normal mode) while I was on my alt DK (Geshrat). I figured we'd complete the run, but the tank ran off. Two other players joined, and left, and joined, and left. You'd think when the PoS confirmation menu shows up people read the "1/3 bosses killed".

Axidd and Alnadr get the big finger for the world to see. Enjoy.
I'd name and shame the mong tank that left after boss two, but I sort of forgot his name. *Beats himself on the head* It was a paladin at any rate. Idiot had a big mouth and ninjapulled both of the two packs after Ick to try and wipe us and then ran off. He died, I did not.
The Nexus
Stormchief from Arathor was another classic Situation-3, though. Sadly he didn't go completely mental, but simply exited the run right about after boss two. He was feeling all big and mighty with his "leet gear", and started pulling no less than 5 packs at once, with a boss at the same time. We had a good healer and survived, but a bit later he decided to pull half the zone near boss three (the tree boss, with slowing trash), which resulted in the healer getting slowed and thus being out of range of the tank. I vanished and survived, but the rest of the party didn't fare too well.
My props to Bryte who was totally cool and as amazed at the tank's immaturity as I was. We then got three replacements and proceeded to clear the instance as normal. With a tank that didn't care about having a green mark on his head.
Violet Hold
Today, a paladin and a druid. (See screenshot below) Both from the same realm.
It didn't take more than a few seconds before the Paladin tank started making trouble about having a mark on his head. Surely the game must be bugged, for there was a mark on his head in a 5 man dungeon. O M G, the world would end. He fell for the classic "I'm not even targeting you" excuse, and we proceeded and killed boss one while he was removing the mark, and I was re-applying it to him. Right around wave 8 he "disconnected" and the druid took over tanking in bear form instead. Just before the second boss we finally managed to kick him out so we could replace him, but the druid who was apparently a real life friend of him left right along with him.
Some people, seriously. It's just a green mark, not the end of the world.

I do SO love messing with their minds though. :)