I had my ignition switch fail in my 85 190E and what happened was the spring load into the start mode let go and although I had all accessories functioning properly, I had no start. I rigged a momentary switch under the dash and used it to engage the starter.

So I know something inside the switch can disable contacts. Also, my current 87 300DT has a sticky switch too but works properly most of the time. I've never had ignition switch problems in any other cars I've owned over 33 years, Just MB's!