You absolutely know that your putt will break a couple of inches to the right. You line up perfectly, make a good stroke, and then watch as the ball rolls and breaks a couple of inches ... to the left! How's that possible?
Turns out, reading greens involves more than just seeing the slope. First, your eyes can play tricks on you. You might be staring so intently at a small area on the putting surface that you don't notice the bigger picture, how the entire green is contoured and oriented. Second, wind can play a role—especially on a gusty day. And let's not forget, mis-hitting the ball out on the toe or on the heel of the putter might produce an embarrassing miss, too.
Another common factor for why a putt breaks in the opposite direction than you thought is grain. The way the blades of grass lie on the putting surface can make the ball roll faster or slower, or pull it left or right. Check to see if the grass on the putting line looks shiny or dull. Shiny indicates the grass is lying away from you, which will make the ball roll faster and move more in that direction. Can't tell if it's shiny or dull? Then look at the edge of the cup. The side that looks burned out is the direction the grain runs. Factor that into your read.