Canni 5 can kill you both directly, and indirectly, as stated by Cattywumpus =).
If you don't have enough HP, of course, it will kill you.
There are those encounters, also stated by Cattywumpus, where low hp agro kills you. Especially when you are battle rezzed and such, and need mana ASAP, but are not yet HP buffed.
And then... there are those times when you canni while being hit and end up killing yourself because the healers can't keep up with the damage intake. For example. the last time my guild cleared killed Ikkinz2, we had 4~5 monsters mezzed in the entrance hall, because of an overpull. I ran up, slowed, crippled, and malos'd them all, then sat down. My guild leader jokingly told me I'd better get ready to tank them ;). We all laughed (I'm notorious for dying on my guild's raids), and I moved back to everyone else. My mana was low, so I clicked my Mass Canni hot key, just as 3 of the mezzes wore off at the same time. 3 kyv's doubling on me for 1200, coupled with the 1900+ canni damage... heh heh, thank the Tribunal for clicky-sticks ;). (I think i died a total of 9 times that raid)
P.S. Hurting yourself has its benefits, aside from gaining mana. In PvP, if you get rooted, canni'ing counts as being nuked, so it breaks root. Just a lil tip ;).