Contranyms: Words that mean two opposites

I asked for examples of "contranyms"-- a word with two opposite meanings. 

Here are some friends contributed:

 Fast= quick to get away/ Fast like "he was held fast by the giant lobster claw."

Bolt= to run away, but you also bold two things together.

Oversight - looking over something carefully -- or overlooking something entirely.
Sanction -- to approve of an action, or to punish an action. 
Weather -- to withstand the effects of weather ("the house weathered the storm"),  or to *show* the effects of weather ('the stone statue was badly weathered")

Cave, as a noun, a big hole in the ground. As a verb, the collapsing of a hole.

Can you think of any others? And why does this happen? One friend reminded me when flammable things were labeled "inflammable" (meaning, uh, flammable-- don't set these on fire). Why do that?