There are only two hard things in Computer Science: cache invalidation and naming things.
-- Phil Karlton
Long a favorite saying of mine, one for which I couldn't find a satisfactory URL.
There is also a variation on this that says there are two hard things in computer science: cache invalidation, naming things, and off-by-one errors.
And for another riff on this:
There are only two hard problems in distributed systems: 2. Exactly-once delivery 1. Guaranteed order of messages 2. Exactly-once delivery -- Mathias Verraes