I lead many LDoN's on my Chanter. Before marking a MOB for a pull, I use Soothe (in the Lull line) to keep adds from coming. Its quite a nice system.
One particular LDoN (and EF, if you'd believe it), my Soothe was resisted at a horrible time, on a MOB in the center of a crowded room. When Soothe is resisted, the MOB may agro.. there's only a chance. It, of course, did. Immediately, I went to start Mezzing adds while the group fought hard to keep back the train.
My mez, which very rarely fizzled, did so.. thirty times in a row until I was OOM. I was so utterly confused and, frankly, humilated, that I was unable to, mentally, face the group again Luckily, the group was a strong one, and the LDoN was still completed in time, despite my bad luck reoccuring during that mission, several times over. I still believe there was a bug that must've fixed itself when I logged, something messed up with the chance to resist or fizzle.. every Soothe I cast for the rest of the mission was resisted, and every mez fizzled.
Since my LDoN's tend to go smoothly, despite the "High risk" constant, I rarely waste time picking out more than a single tank and a single healer. Which is, normally, fine.. single pulls with everyone buffed up is easy for a group, regardless of having a single tank/healer. Anyhow, point being, I've had tanks go LD before.. I've even had some rather apathetic ones simply camp, without notice.
The strange trend in this case is that, after a tank leaves, the healer seems to suffer the same fate (or apathy), and the group's without either. (Which is a time you're thankful you got Temp before hand and have your Int-caster butt act as a tank, telling the group to nuke with every mana they have to complete the mission in time.)