Sometimes, defenses show 1B, -1B or -2B as damage dealt instead of the correct value. I have seen it most often occuring with Ramsters though I have also seen it on others. It does not seem to occur to towers which see no action. Sometimes (rarely) the value returns to normal after dealing more damage (and the value appears to be accurate). I do not have a reliable method for reproducing this short of just placing lots of towers and waiting.
Perhaps the first couple bits (or more) of the the display value are getting overwritten? That would explain seeing both negative and positive large numbers, and the number sometimes returning to the correct value. Repro Chance: 40% Steps for Bug Repro:
As Abyss Lord, set up many Ramster towers in lanes.
Inspect the tower stats (specifically damage dealt).
Damage dealt shows as 1B, -1B, or 2B.
Rarely, after dealing more damage, damage dealt returns to normal.
Expected Results: Damage dealt should show the correct damage dealt, usually in K or M, and should never be negative.