And off by one errors

That's a nice addendum to the quote, actually. Will use it in future.

> There are only two hard things in computer science: Cache invalidation, naming things and off-by-one errors

And dates and times And currency

Manage date and time in UTC/Zulu. Append the time zone if that meta is needed and store both the client and server for backend. That way you don't have to deal with time travel and can handle time shifts.

I would say that concurrency is a caching issue once proper locking has been set.

And ce conditions rainduced by concurrency.