I voted slow Block leak.
My only real problem that I have had was over the course of 1 1/2 years of moving my pump around while still connected to the water block (bad idea, I know) the poly top of my TC-4 cracked and began a slow, unnoticeable drip onto the video card. On several different occasions, I would be in the middle of a game and the whole system would just lock and would refuse to POST. I would shut the system down and pull the video card out to discover that the whole AGP socket was filled with water and one component would be corroded. Clean out socket, clean up card, scrub off corrosion, dry out socket, and put everything back together and it was good to go for another 2 months or so.
Last time that happend though, the corrosion finally ate through a connector on the back of my R9700Pro and the card wouldnt work anymore.
SO, ordered all the parts of my PCI-e system. Luckly, while I was waiting for the new video card to come, I was able to resolder the missing connector and the video card still works (I got extreamly bored
)
Moral of the story: will never buy a poly top water block ever again
