I know that the stamp cost has increased about a month ago, but last week I forgot and sent some envelopes out with the old stamps on them. What will happen next? Will I get the envelopes back or will they get lost somewhere? Has anyone had this experience?

Either you will get them back or the recipient will pay the difference.

One of 2 things happen, either the letter will get delivered with postage due from the receiver, they have the option to pay the difference or return to sender.

Yup..

I thin you'll get them back