A few days ago I had the great fortune to spend a day at Langen Radar to look over the ATCO´s shoulders and ask questions. Highly recommended! Anyway... I talked to this one chap, and he told me about an F-16 of a certain european airforce he was working a couple years ago (as Rhein Upper I believe, but don´t hold me to that). He cleared the F-16 for FL270, and the F-16 climbed with around 4000ft/min. It approached FL270, went past it, kept climbing through FL280, 290, 300, 310, missed a 747 by just half a mile, kept climbing and finally levelled out at around FL340.
While the ATCO was still busy recovering from his heart attack, the F-16 pilot keyed the mike and said "ooops, sorry for that level bust".
Level bust?
Understatement of the year.