So, if it's accidental, it means it's an error. And if it's an error, it means it's a bug. Or am I wrong in understanding what a bug is?

No, I mean that maybe the game developers wanted the ballista to function in battle the same way as a tent, but a very clever programmer accidentally added a window to the ballista that displays the number of shots, even though it shouldn't have been there. And here there are two different understandings of the bug. First, the bug is that the ballista's shots go into negative numbers. Second, the bug is the presence of a shot counter window on the ballista, while the ballista works correctly with infinite shots