I had water pumps go on two different W126 cars. One of the car's leaks started as a slow drip, which I later found was coming from the drain nipple on the bottom of the water pump. You'd have to use a mirror to look at it, or put your hand under the water pump and feel it. I couldn't figure out where the leak was coming from, until I was driving one day and the water pump just gave out and the coolant gushed out.
I've heard of water pumps going frequently in W124's - its a little involved to replace it. I've done it on a W126 with a friend - it took us about 1-1/2 days but never did it before. The W124 requires the radiator removal from what I understand.
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino