Sometimes i write do not bend but when i not, nothing bad happen

I never write DO NOT BEND and haven't had trouble except once. Also I doubt the mailman would read the envelope.

I personally wouldn't worry about it. I actually never write 'Do Not Bend' on either of the envelopes and I have not had any problems thus far.
I have heard from different people who wrote 'Do Not Bend' that they have had problems, as well as people who don't write it. If it would put your mind at rest then just ensure that you write it in future, but definitely don't let it play on your mind until if/when you receive a success!

I wrote some fan mail days ago to a celebrity, and I wrote "Do Not Bend" on my SASE and sent along 2 index cards. But I didn't write it on my main envelope to the celebrity, and i'm worried that the postman could bend the celebrity's envelope. Has anyone wrote "Do Not Bend' on their SASE and not on their main envelope?