Commit

I find it very tasteless and rather irresponsible when a person has voluntarily committed to do something then backs out.
I was raised not to volunteer for anything if there's the slightest chance I can't deliver.
I understand that circumstances arise where it's almost impossible to deliver on your commitment. In that case I have been raised to come up with Plan B. This way nothing gets thrown out of whack.
To volunteer for something then give a lame excuse why you can't deliver is down right tasteless and makes me think lose trust in you and your character.