PendingTicks aren't processed outside of simulation distance. This can allow players to keep generating more and more pendingTicks in a chunk outside of simulation distance until the game crashes whenever that chunk is loaded. It then would require a third party program to remove. This however takes some time. The most noticable effect would be an incredibly large world size.

Players could encounter this while being AFK at a farm and not realising that they are still half-loading another redstone contraption, which could easily produce pendingTicks and make the world unusable.

I would expect the game to stop the amount of pendingTicks scheduled from exceeding a certain amount, or for pendingTicks to be processed outside of simulation distance.

Steps to reproduce:

1. place lots of dispensers containing a lava bucket on the edge of simulation distance (more dispensers means you generate them faster, so do lots to speed things up)
2. put them on a fast clock
3. afk for a while and then note the large increase in your world size

There are other ways to produce pendingTicks, but this is one of the simplest.