The cancellation can be with non-visible ink, since mail is processed by machine for almost the entire process. So it may detect the stamp was previously used.

Notice the bright orange bar codes on the back or front of some envelopes -- it was machine processed at the USPS side (versus pre-sort from the mailer side).

I'll take my chances.

It is illegal to do that. You know that, right?

Anonymous wrote:If you get a letter in the mail and the postage wasn't cancelled, do you take the time to reuse it (with glue) or do you toss it?

I would use the stamp again (and have done so in the past) -- but it's been ages since I've received a stamped letter, much less a stamped letter that was not properly cancelled.

Anonymous wrote:How often does that happen?

At my office, about 3 days/week.