UA will likely make you sign the waiver on the back of the luggage tag to say that you accept liability if it's damaged due to it being a box.
In my experience, in about 50% of cases they will then proceed to give you the signed liability statement, so they have absolutely no proof that you signed it.
Go figure...
As others have said, the TSA may open it and re-seal it in the same way that they can for any other checked luggage - I've had that happen twice out of maybe 8-10 times I've checked a box.
Originally Posted by
StingWest
I will bring extra tape in case I have to re-do it for the trip through London on the Tube!
I normally just put an extra roll of tape in the box, that way if I need to open it for any reason the tape is there ready to go.