How often does it happen to you that you create something or edit something, you finish it, and then you decide “no that’s not quite right”?
Consider a destructive operation like a bevel using 
Before you finish the operation this collapsed panel appears: 
When you expand the panel you are able to specify all the possible values:
Once you click somewhere in the viewport, the panel disappears. That’s it. If you want to change any of those values you will have to Undo 
and create the bevel again.
Unfortunately what is done is done particularly if it is a destructive operation.
Or is it?
Here’s the Tip:
You have one last chance once the parameter panel disappears. Provided you don’t perform any other action or create anything, if you press 
, the last parameter panel pops up again allowing you to modify your last action or creation.