Found an interesting bug with Chryssalids and the Evac-based missions (such as 'destroy the facility' missions)
Speccifically, the chryssalid spawn your venom creates counts as a non-evac'd soldier, and as such will prevent you from ending the missions if it doesn't have a 'force quit' timer on it, and leave you unable to exit the mission without simply quitting and re-loading from an earlier mission.