The head gasket material...
... OEM was defective, but Mercedes will never admit to it. This is a very common problem on 124's, including mine, which I have yet to repair.
They now use some super high tech space age sealant to seal the head onto the block.
Other reasons mentioned for this failure rate is an aluminum head attached to a steel block, which was a poor design I suppose.
Be sure to have your valves done "while you're in there", so you won't have to revisit the process of taking off the head at a later date for valve problems.