Geography. For some reason I decided to make my characters walk from Nevada to California.
I know I need to research California plants; luckily we are members of a great botanical garden and I am going to pump the docents and my green-thumb mom for the info.
And just this week I came across a passage in my novel with detailed descriptions of an archer's bow and arrows and wrist strap... and it sounded great and real was all just made up.
I yelled to DH, "Oh great, I have to research ARCHERY now?!" I tend to just write whatever and then go back and research and change things. I haven't written myself into too bad of a corner yet.
I like Ann Lamott's chapter in Bird by Bird where she talks about researching and how people will ask her about loving to garden, and she tells them she doesn't garden, just talked to gardeners a LOT and wrote about it really convincingly. But people assume she really does garden!