It was supposed to be one of those D-Day (or was it VE Day...?) anniversary deals, where he was visiting some graves of people who had died in the war. Nobody thought it important to tell him that he was laying wreaths at an SS cemetary. Oops.
Nowhere near as entertaining as the "Clymer" incident though...
It was a military cemetery which contained the graves of Waffen-SS, among others. Pat Buchanan is credited with being influential in the decision, the visit wasn't originally scheduled.
As his speechwriter at the time, he's also credited with Reagan's line delivered at the cemetery, that they were"...victims just as surely as the victims in the concentration camps."
Surprised? :>)