That and off by one errors ;)

Concurrency

You forgot 2) Cache invalidation and 3)

Haha, you are correct, but there are some other hard problems that I now remember after some quick googling since i forgot all the related jokes :)

There are only two hard problems in distributed systems: 2. Exactly-once delivery 1. Guaranteed order of messages 2. Exactly-once delivery

There's two hard problems in computer science: we only have one joke and it's not funny.

Source: https://martinfowler.com/bliki/TwoHardThings.html

The two most difficult things in software development.

    * naming things
    * cache invalidation
    * off by one errors

It's always worth having it stated in the canonical form. :)

The fifth hardest thing is figuring out the canonical form :)

I’d risk saying we’re all intensely aware how hard Cache invalidation is, right now

well, we can only speculate...