“Do Not Bend” is an instruction to the mail carrier, not the recipient. It’s just a way of telling everyone along the delivery chain that there is something inside the envelope that may be damaged by bending. There’s no punishment for bending something with “do not bend” on it, though a commercial carrier (like UPS or FedEx) may be liable for damages in certain circumstances if they destroy the contents.

Apparently nothing. It seems to be used as a challenge for the delivering company to see how many different ways they can twist the envelope.I think that it was originally used to designate that there were probably photos being shipped. Now that we no one longer have photos sent out to be processed, it seems to mean very little!!!

You could destroy the photographs or other coated items within.

It is a challenge to the mailman.
Just like the labels “FRAGILE” and “THIS SIDE UP”.

You mean like this diploma?

It just ruins whatever is in there.

The worst case scenario would be a crease in the space time continuum opening up a crooked line all the way from your letter box to the constellation Orion. The consequences could be alarming particularly if the mutant zombies of Zone X find out about it. Unlikely to happen. I wouldn’t worry about it.

In the instance described by the OP, I’d say the words “DO NOT BEND” was a advertising ploy to get the envelope opened without just being chucked in the recycle bin.