What happens when you divide by zero. [edit: apparently the image isn't there any more; I didn't realize that would happen. The image was in the style of the Successories motivational posters or parodies thereof. A man stands wading in a body of water, not from an apparently infinitely deep whirlpool. Underneath, the text says:
YOU SON OF A BITCH
You divided by zero again, didn't you?
It's possible I don't have this exactly right; I'm doing it from memory.]
[edit again: Mark Dominus pointed me to another source for the image, which is linked to above; I think they allow hotlinking, but in case they don't, see http://macrochan.org/get.py?sha1=QOMEULLMJLINOMIA4L2FUAEQNHRMCC7O. Several dozen images on the same theme are available here.]
One can only imagine the more serious consequences if one, say, takes square roots or logarithms.
It's quite fortunate that doing incorrect mathematics does not actually have disastrous consequences in the physical world. Otherwise the area around any calculus class would implode.