I voted multiple incidents!
1.Pump blockage
I replaced an Old Aquaclear powerhead/big res. with a modded Mcp350/small res. (6800U is hooge!) My temps were abnormally high, I opened the res. to find a trickle flow.... Mcp350 got clogged by bits of Radiator crusties that the Aquaclear readily pumped!!
2.Hosebarb leak
I've had very minor drips from "not tight enuff" on both 9500np and the new 6800Ultra!!! drying cured both cards.
3.Radiator fan
One of my YL 120's stopped spinning, peeled-up the sticker, put more lube in, she's spinning like a champ! I was so suprised to find my shamefully cheap fans to be popular for WCing
I finally cracked my WCing open after 21/2 years of running.... Bio nasties, crazy galvanic corrosion re: WW Alu top, and chromed hose barbs. It had no leaks! Thanks to the O-ring sealed barbs.