> But if you aren't writing a compiler or an embedded system, you should never do this.

I have this hex trie library that uses this to differentiate between a leaf and a node that will soon be optimizing single-value nodes as tagged pointers. __edit__ umm... single-value nodes are leaves!?! Yeah, not enough coffee apparently.

Not to mention the tinyscheme interpreter I occasionally poke at that TFA gave me a bunch of ideas to try out.