We made gnolls in the mines yesterday, and a funny thing happened: we were already on our leave, and transited from the lake area to the barricade area already, when the characters suddenly bursted out in fire. Apparently, the fire-throwing ballista hit us, even effecting us in a totally different area.

That ballista already produced other interesting effects at other times, like keep firing even when the trap was avoided alltogether, firing on characters when already all the gnolls were taken out, and the characters walked toward the tunnel leading to the boss area.

The script would worth a look.

I'd add that I've seen delayed trap response in other areas as well, with traps being triggered and the effect following a relatively long period afterwards.

i always considered it lag =S

These projectile traps do have issues in lag, as far as I know.

its just very sticky dust (from the projectile) which bursts in fire after some unknown duration... ;)

Likely the same issue as here.