Here's a possible scenario in which this could be re-created, but we'd need some volunteers.
I notice that blue team has been eliminated...
Thus it's possible you made the jump to cap AFTER or AS the round ended.
As many know, if you select change teams and don't die, as soon as the round ends you take on that jersey.
Is it likely, this guy switched to observer, the round ended by blue team being eliminated, since he was on red he switched to observer and then you made the jump, thus hitting the solid observer?
If anyone wants to test this with me, grab a server and pm/irc me, I think it'd result the same