1. A puddle on the ground that you don't notice until you've stepped in it.
2. A puddle that seemed shallow until you stepped in it. Turns out that it's not shallow at all. You end up ankle-deep in rain/piss/vomit, etc.
May: "Why the hell are your jeans soaked to your ankles?!"
JJ: "I stepped in about seven ninja puddles going across the campus."