This is from one of xkcd's recent comics, talking about stopping people dead in their tracks with intriguing problems... "nerd sniping". It's about on the same level of effectiveness as a weapon as the world's deadliest joke, which even has a mention in Wikipedia. Going back to the comic, the problem of an infinite grid of resistors is apparently from the Google Labs Aptitute Test, which I looked up. The questions were harder and more ingenious than I thought they would be, especially the numeric-acoustic ones.
I remain, however, one of the people who are easily nerd sniped (computer scientist for one point?). Here's an example. I thought about the actual question for a bit, using my rusty physics skills, and could only narrow it down to the resistance being larger than 0 but smaller than 3 (since the two nodes are on a parallel circuit, and the resistance has to be smaller than the smallest resistance).
There's a fairly popular flash game called Desktop Tower Defense, a strategy game of sorts. I played it for a bit, then did the "fun" version where you start with 10000 gold. I spent the next few hours (or days) trying to create as long a path as possible for the creeps. That would mean more towers and more time for me to deal damage. I think my last version lasted up till level 40+ before I had to upgrade the towers...
Clearly, this is one of the xkcd comics which real life should not imitate.