Here's a cool piece of bitcoin script lore about an obscure difference between two ways to use timelocks:
"height-locked transactions are standard before they’re valid, but timestamp-locked transactions don’t become standard until they’re valid"
Source: https://medium.com/summa-technology/the-bitcoin-non-standard-6103330af98c
This means bitcoin has two different types of timelock errors it can throw. It can say "your timelock isn't passed yet, so your tx is invalid" or "your timelock passed according to your system clock, but the network hasn't confirmed this, so your tx is non-standard"
It seems weird to have such a similar error be part of the validity check AND the standardness check. But as pointed out in the post I linked, this is because "transactions that are still time-locked are not just invalid, they’re also non-standard."
