Murphy's Law is an old adage along the lines of "anything that can go wrong, will go wrong."
Murphy's Rules is a cartoon column first published in the November 1981 issue of Space Gamer, published by Steve Jackson Games, and intermittently continued into the publication of their Pyramid magazine. The column itself was all about illustrating sillier rules in games and how they broke with your expectations of how things should work. As Steve Jackson himself explains in the introduction to the compilation volume (which you might consider checking out), this isn't really a terrible thing and there can often be perfectly valid reasons for rules failing to match up with reality ranging from ensuring playability to being a product of obscure interactions to simply being due to the fact that no rule system ever created by humans can model the universe with 100% accuracy at all scales. If the rules produce odd results the question is less about if it's good or bad and more about if it's funny.
Making fun of rules is a long-standing nerd tradition even in the present day- the Giant in the Playground forums have been collecting dysfunctional rules for years now with the first thread dating back to 2011, and I've certainly taken inspiration from it. Back in February 2013 the Murphy's Rules thread was posted to the Something Awful Traditional Games forum and people would post various amusing, dysfunctional or otherwise odd rules interactions.
I was one of those posters, and while I started simply enough my earliest posts in the thread are probably not the things most people remember. If you were pointed towards one of them, it was probably one of my "effortposts" wherein I'd take various RPG rules and thoroughly run through them in order to create the most absurd scenarios possible while still remaining within the rules as best I could (partly inspired by Randall Munroe's "What If?" column over at xkcd.com wherein he'd answer odd theoretical questions using real physics).
You'll probably notice that many of these (if not most of them) use the 3rd edition of Dungeons & Dragons and/or the Pathfinder Roleplaying Game, and I've got several reasons for that. I'm familiar with both, they're prolific games with a large amount of rules (and thus lots of moving parts and places where things can fall apart when combined with other elements), sites like the PFSRD make it easy to show my work, and perhaps most importantly there is a sizable percentage of the community that likes to think of the rules for these games as something that models the mechanics of the game world itself and thus mechanical interactions are also observable truths of the world. I am not one of those people and I believe that when the d20 ruleset approaches reality it's frequently by accident, but I do find that treating game mechanics as world mechanics makes it far funnier when it does something it shouldn't do and it creates some entertaining interactions when characters start operating under the same ideals that players can have.
When mechanics are physics, munchkins are engineers, so have fun and don't take this too seriously... I know I will.