When chunks are loaded, two major issues occur. They both are caused by chunks being reloaded so I am putting them in the same ticket.

A) Any entity positioned near the edge of a chunk has a high chance of disappearing. This commonly causes villages and animal farms to lose all of their mobs, and makes gameplay nearly impossible.

B) Redstone that was powered will not always save it's powered state, causing Redstone contraptions to break. This makes it impossible for developers to create maps that use Redstone clocks of any kind.