Hoist with one's own petard...

Hoist with one's own petard - idiom, to fall into one's own trap, to have one's plan backfire... to be injured or killed by one's own weapon.

Believed to be coined by Shakespeare in Hamlet when the Prince of Denmark ensures that the death warrant carried to the King of England by his schoolfellows, Rosenkrantz and Guildenstern, bear their names, not his:

There's letters seal'd: and my two schoolfellows,
Whom I will trust as I will adders fang'd,
They bear the mandate; they must sweep my way
And marshal me to knavery. Let it work;
For 'tis the sport to have the enginer
Hoist with his own petar;

I ran into a fabulously funny example of this yesterday whilst talking to a good friend online. All names have been changed to protect the innocent...or, rather, my friendship:

Teacher: I got shafted today in Year 12's lesson
Irim: what happened?
Teacher: a few weeks back Jasmine and Susan were looking at a newspaper
Teacher: I told them to stop and pay attention and they did
Irim: carry on...
Teacher: I said something like, "And if you are going to bring in a newspaper at least bring a quality paper, not something you get free on the tube...."
Teacher: Jasmine: "Like the Mail, sir?"
Teacher: Me: "Find a dictionary; look up 'quality'"
(Irim's aside: When reading this last line, who else had the same feeling that one gets during a horror film when watching someone go into the dark basement alone? Yeah, thought so...)
Teacher: Today.....
Teacher: the newspaper re-appears
Irim: ok
Teacher: Me: ".....and this argument is valid but unsound because...... erm, Jasmine?"
Teacher: Jasmine: "Sir?"
Teacher: Me: "Lose the paper"
Teacher: Jasmine: "But sir, it's the Telegraph.... pause"
Irim: uh oh...
Teacher: "Britain's biggest selling quality daily......"

Absolutely priceless. I couldn't stop laughing...it's exactly how I used to be with my kids.

And as for Sir? I told him he had been hoist with his own petard. His response?

Teacher: indeed
Teacher: but pleased I'd got her to buy a good paper

A teacher till the end, then.