When I was a kid, I used to buy the cacheted envelopes for certain stamp issues and service first day covers myself. Every once and awhile companies like Artcraft, Artmaster, etc., offered a packet of cacheted envelopes that were over-runs of past issues. I used a few with the wrong cachet for some stamps, merely as an oddity; the rest could be used for general mail, or in my case, I put several away in a box for future use -- but never used them. In fact, I think I still have a few lying around!
There may be some that might be worth a bit more than normal. For example, Canada Post issued a stamp in 1979 honouring the author Frederic Grove. Unfortunately, the portrait on the cachet was not of him. The error was realized shortly after issue, and the covers were recalled. The current Unitrade value for this cover is about 3 times that of any contemporary cover, which is still not a lot.
Similarly, in 2009, Canada Post issued a stamp honouring the Silver Dart, the plane that made the first flight in Canada. At the time, there was some discussion over whether the plane pictured on the cachet was in fact the Silver Dart. I`m not sure how that was resolved, but the covers don`t seem to have any extra value.