I've noticed my triggers sometimes failing to attach player with triggerAttachVehicle. So attachedTriggerObject would be <obj-NULL> and instead of "VEHICLE" the trigger activation would be "NONE". I narrowed this down to mission being read from cache.

This only fails for the 1st player who is the one starting the mission. After role assignment before briefing there are 2 progress bars on the title picture, one long at the top and one small right above mission picture. If everything goes well after both bars fill it switches to briefing very quickly. However when it fails, it takes quite some time before briefing displayed.

This happens often if I just quickly restart server and join it. If however I change something in the mission and my client is forced to redownload it, it doesn't happen. This leads me to think that it is somehow connected with mission read from client cache. Sorry I cannot be more specific, but this is really a game breaker, because if it can break script commands like this, who knows what else it breaks.