My girlfriend and I did some work on her bike a while back (DF, single speed) which required removing the seat and post, among many other things. After we put it back together, she reported that the steering was weird. I tried riding the bike and found that it really wanted to weave back and forth in an unsettling way. After looking at it for a long time and going over every part we had touched, I noticed that the clamp on top of the seat post was flipped around, so the seat was inches farther forward than it used to be. I flipped the clamp back around and the bike handled like normal. Conclusion... bikes are scary magical things.