Things that seem "simple" but end up taking a long ass time to actually deploy into production:

1. Using a new payment processor:

"It's just a simple API, I'll be done in 2 hours"

LOL sure it is, but testing orders and setting up a sandbox or making sure you have credentials right, and then switching from test to life and retesting, and then... fuck

2. Making changes to admin stats.

"'I just have to add this column and remove that one... maybe like a couple of hours"

YOU WISH

3. Anything Javascript

"Hah, what, that's like a button, np"

125 minutes later...

console.log('before foo');
console.log(this.foo)
etc..