Look at this bad valve from 190E 2.6
 

Hello. Here is a picture of the bad valve that I took out of my 89 190E 2.6 with only 104,000 miles. What exactly would cause it to do this?

-Andy

I am an aircraft mechanic by trade and also have built and repaired several oval track race engines. By looking at your valve, it looks like some debris lodged itself onto the valve causing it not to close completely. As the hot gasses from the combustion process escaped by the valve, they burned the section out of the valve nose. With the stellite or otherwise hardened valve seats, they would go relatively unaffected. There's really nothing your could have done to have prevented it from happening unless of course you were running lower octane fuel than premium.

D.McCarroll is rigth, is there any chance you had been using some kind of injector cleaner before that happened.

How does the valve seat look?