Yes, this has happened to me several times !!! Even though I am aware of the problem, I have still done it. I don't know if it's my age making me forget about it again, or whether it's because it has been inbuilt to my brain, that cellophaned cards contain an envelope. Another thing I have noticed is, on the back of individual cards, there is an envelope symbol saying small or large letter postage rate. When I checked the size at the Post Office, I found a lot marked as large, actually fitted in the small space. Therefore I have been done on two counts !!!!