I understand that zombies endlessly spawn. However, scoreboards seem only to register to round 99. Does this mean that there is a glitch where you die at round 99? Or is it simply a issue where the scoreboards can't record anything over 99?
There is no end to the zombie mode, and people have been to at least level 100. I would wager that most people who have made it past round 99 glitched or cheated though (like the guy in the video), as the difficulty at such a high level is nigh impossible, and it would take an entire day or more of dedicated playing to get this far.
The leaderboards weren't really designed for this kind of play, so it is highly likely that the max level they can display is 99, and that the people at the top glitched to get there.
I have been to round 147 and I was just fine, but the reason for this is because there is automatic target finder (so you aim for the zombies and makes aiming easier). I also did it tactical, was smart about my moves, trained a lot, and with monkeys and the crossbow upgraded it attracts the zombies and kills them or makes a crawler.