Yeah lemme explain.
There was one guy that looked like he was on top the building, but there were also shots coming from the window near the top. The building I'm talking about is the large one between the Train Accident spawn point and the Market spawn point. It's the tallest one in the area.
I'd have to doubt that it's the old tank spin glitch because as best I can remember my team had both tanks all round and I know we had the APC the whole time because I drove it.
The best starting point I can offer is that it seemed like they did it with a vodnik, the round after I saw one with all four of the glitchers in it driving near that building.
Oh also for the record, Friendly Fire was on in this server, so they didn't use C4 to launch themselves to the top, they would have had teamkills for that.