Why shouldn't we create our own security schemes?

I see a lot of questions around here about custom crypto and custom security mechanisms, especially around password hashing.

With that in mind, I'm looking for a canonical answer, with the following properties:

• Easy for a newbie to understand.
• Clear and explicit in why rolling your own is a bad idea.
• Provides strong examples.

Why shouldn't we create our own security schemes?

I see a lot of questions around here about custom crypto and custom security mechanisms, especially around password hashing.

With that in mind, I'm looking for a canonical answer, with the following properties:

• Easy for a newbie to understand.
• Clear and explicit in why rolling your own is a bad idea.
• Provides strong examples.

Obligatory xkcd.