Occasionally on Bedrock realms, a pillager will spawn alone and, upon it noticing and targeting a player, the realm will crash. The pillager does not need to be hit by the player, it just needs to notice the player. Typically this results in the world getting reverted to a previous save, losing everyone's progress. Unknown yet if this happens on singleplayer.

Reddit r/MCPE thread about this bug: https://www.reddit.com/r/MCPE/comments/dxyqdc/punching_illagers_on_patrol_sometimes_crashes_my/