But seriously :

The greatest mistake you can make in this life is . . .

Disrespect others & Disrespect their opinions & Disrespect their beliefs,

I think you must go further, for example:

“The greatest mistake you can make in this life is to continually fear you will make one.”   any things.

The greatest mistake you can make in this life is . . .

To sleep in the same place with someone ate alot before sleeping :D