In my initial days as a web developer, i was assigned a task, to implement a cart share functionality in an e commerce company.
I made the functionality and tested on my system.
Result: working good.

Pushed it to beta testing environment.
Resilt: working good.

Pushed to pre production environment.
Result: working good.

Pushed to live site.
Result: 😀 Error in live site..
So a call comes to me from my team lead..
Asks what was the issue...

Me: i dont know either.
....

After 3-4 hrs:
I found the reason.
My system, beta test env, pre prod env are all having latest php version (5.6 i guess)
But the live server had old version of php.

Me: laughed like anything.
I didn't know that these things would matter in such a great level.

Moral of the story:
Be one with the force (server in this case)