Witnessed it on a random Twitch stream, related to Gray Firetank event.
A streamer lured the firetank to Ariral Treehouse, where one of arirals with a blaster spawned to destroy the tank. After the tank was destroyed, the cloaked ariral kept on running around the treehouse terriory, until the game gave an event-related error and ariral disappeared.
Message in question:
Debug message
Warning: the object ariral_shooter_C_2147199161 has detected a negative event!
Report to the bug forums your sequence of actions!
Event variable has been set to 0
A link to a clip where error occured (error message pops at 3:08) Confused KelLyss drags Firetank to arirals, which in turn confuses arirals - Twitch
Likely game version is 0.8.2b