mmm that sounds like completely bizzare behaviour, i thought it might have been a shared material or texture or script between them ,but adding a random entity to it takes away your crash , makes no sense to me , except what sounds logical is a memmory related problem as you mention , or an engine bug , have you tried out asking the dev's .

i mean what would adding/removing a random entity effect in the level itself before loading , all i can think about internally ,during loading ,would be the entity list , memmory ,pointers , shedule list , materials , textures , bounds ,hulls , stack sizes perhaps , the list could go on but only the devs would know what is internally effected.