Sometimes, when I blurt out something stupid and inappropriate ("That fireplace looks hideous! What are you going to replace it with?... Oh ... you like it and want to keep it as a feature..."), or do something really dumb (like call out an emergency electrician who charges you £70 to flip a switch), I wish there was a CTRL-Z* that you could simply tap to Undo that mistake.
It would just take you back a minute or two and give you a chance to do it again - but do it right instead.
As most of what I do is computer-based, I'm so used to doing it throughout the day I forget you can't actually do it in real life. Shame...
*or CMD-Z if you're a Machead like me.