Do not confuse your users: Good labels are important for a better user performance and to increase usability.

Quick recipe for writing error messages: inform (1) WHAT went wrong, (2) WHY it happened, and (3) HOW to fix it.

by Filipe Levi on tweeter

The chance of someone be blocked by mistake in this screen is very high. Since the distance between the actions of Accept and Block has only a few pixels…  a small slip with the mouse and you unintentionally will block someone.

Error prevention
Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone conditions or check for them and present users with a confirmation option before they commit to the action. (From Nielsen’s usability heuristics)

While some displays stop working with a small hit or fall, this poor Nokia cellphone display continues working perfectly.

Probably there was no diversion sign with a right arrow =P
I know diversion signs are quite self explanatory, but that was funny.

Aboard a meticulously restored 1920′s biplane. San Diego, CA.

Warnings are one of the common strategies for incorporating forgiveness in design.

Forgiveness in design helps prevent errors before they occur,  and minimize the negative consequences of errors when they do occur.

Road signs make roads more forgiving by warning drivers of impending hazards.

(From Universal Principles of Design book)

Sign at Barreira do Boqueirão beach in Japaratinga.

Trying to keep the feet away from a flooded street in Recife.

I don’t even know how to use a parking meter, let alone a phone box.

Princess Diana