Remember the boss from last month's entry, who wanted you to rush rush rush and hop to it on his supposedly need-it-now task, which rarely really is? That kind of boss is likely to also pile unreasonable amounts of work on you, especially claiming again that it's an "emergency".
Don't fall for it, unless of course it's a true emergency. That means, something not only unforeseen, but unforeseeable. It's a manager's job to plan for the things likely to go wrong, allow slack time for them, and recovery time afterward.
If you give in, even once, you set a precedent. The boss now knows he can count on you to work your buns off, probably for little to no recognition (let alone actual reward), on a whim. He will expect you to do it. If you don't do it next time, you are "not meeting expectations". Never mind that the expectations are utterly unreasonable.
There is an old military expression about this: "Take more than your share of objectives, and you will be given more than your share of objectives to take."
It can be tempting, as so many short-sighted companies pay all glory and homage to the fire-fighters, but slight the diligent workers practicing fire-prevention. In the programming world, this shows up as support for, for instance, 80-hour weeks of debugging, versus 40-hour weeks that include good unit test coverage.
Repeat after me: "Your lack of planning does not constitute my emergency."
Okay, putting it that bluntly may not play well with the boss, but if you get fired for sticking up for yourself, well, would you want to work for such a jerk anyway? (By the way, one of the first jobs I ever quit, I quit partly because the boss was a jerk.)
One last note, lest some of you make some unwarranted connections: no, I have not personally worked for a boss who was all that prone to pulling this particular stunt. It is not why I am leaving Comcast, nor is my boss anywhere near that bad. But it is one of the canonical "Stupid Pointy-Haired-Boss Tricks" (with no apologies to either David Letterman or Scott Adams).