Not sure why it's "unusual". Either 1) someone's fancied flying a bit high or more likely 2) the Mode C/S is a bit out of kilter.
Kid of reminds me of all these UFO reports. "On xyz date, Controllers spotted a number of blips (blips?!? WTF? Usually a BS giveaway) on their Radar display moving at 5000mph at an impossible height." Any radar engineer knows that all sorts of s**t appears on displays that isn't there because the tech is imperfect to say the least...
jayteeto - Were you piloting Airwolf?