Vapor/Charcoal Cannister

The charcoal canister can fail in two ways.
1) it can become Saturated with every carbon atom having fuel attached to it., and
2) The carbon starts to leak out

As to the former the canister gets very heavy. Theoretically the fuel vapors should get pulled out every time the engine is running and warmed up. It doesn't always happen, as the controls and/or hoses can fail, or the dust filter at the bottom gets plugged.And I suppose there may come a time in the life of the carbon that it it just fails to let go of the vapors.
As to the latter, you almost never hear of that happening.
I have heard of them becoming moisture-laden,tho.