1 {scene: Ophelia is at Shakespeare's desk, while Mercutio looks on}
1 Ophelia: Think of it another way, Will. You have something you want to do, like track Malfoy, and something you need to do, like Hogwarts homework.
2 Ophelia: Now think. Catering to your own selfish desire, or fulfilling your greater responsibility? What would Harry Potter do?
3 Shakespeare: Get Hermione to do his homework for him! You're a lifesaver, Ophelia! I'll send you the project documentation files!
4 Ophelia: {walking back to desk} <sigh>
4 Mercutio: {walking with her} Petard. Hoist.

Hamlet, Act 3, Scene IV:

HAMLET:

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 engineer
Hoist with his own petard: and 't shall go hard
But I will delve one yard below their mines,
And blow them at the moon: O, 'tis most sweet,
When in one line two crafts directly meet.
This man shall set me packing:
I'll lug the guts into the neighbour room.
Mother, good night. Indeed this counsellor
Is now most still, most secret and most grave,
Who was in life a foolish prating knave.
Come, sir, to draw toward an end with you.
Good night, mother.

Hamlet here is referring to his old schoolmates Rosencrantz and Guildenstern, with whom he is travelling, carrying letters written by Hamlet's evil uncle Claudius (King of Denmark) to the King of England. The letters instruct the king to execute Hamlet - a plot which Hamlet suspects. Hamlet does not trust R. and G. (as adders fang'd) because they bear this mandate to marshal him to knavery, i.e. death. However, Hamlet has a plan. He will secretly alter the letters to refer to R. and G. rather than himself. Then:

Let it work; For 'tis the sport to have the engineer Hoist with his own petard:

A petard is a small medieval bomb, used for blowing holes in castle gates and walls during sieges. You may recall one being used to spectacular effect during the Battle of Helm's Deep in the film of Lord of the Rings: The Two Towers. For an engineer (i.e. a military fortification building and breaching expert) to be hoist with his own petard is for him to be blown up by his own bomb. In other words, for his destructive plan to backfire disastrously.

R. and G. are indeed hoist with the petard of their poison letters, as the King of England immediately takes them prisoner and executes them as soon as they give them to him, and Hamlet escapes scot free to return to Denmark and plot the downfall of Claudius.

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2016-11-02 Rerun commentary: I had to reread Will's line a couple of times to understand it just now. He means, "I'll send you the project document files to work on. Instead of me doing them." I hope this was clearer when it was first published!

It is a bit odd how the King of England takes the letters from Rosencrantz and Guildenstern, reads, "Please execute Rosencrantz and Guildenstern, signed, the King of Denmark", and then goes ahead and immediately executes them, without so much as a, "Hmmm, an order to have themselves executed is a slightly fishy thing for someone to give to me."

It's like people handing him orders to execute themselves happens every day.