expect the expected
When people waste time planning for the unexpected and unlikely and totally overlook the obvious, usually looking like a dumb-ass as a result.
Geek: "They said 10% chance of rain today so I packed an umbrella, raincoat, and boots, but forgot my sunscreen. It was hot and sunny all day so I ended up with a wicked sunburn."
Gal: "Expect the expected"
Gal: "Expect the expected"