The ultrasound can only ever give up to 95% accuracy, which means at least 1 in 20 gender guesses are wrong, but actually it's probably more common than that.

Why not just stick to gender neutral clothes and colours. That's what we've done, despite believing since week 19 that we "know" what we're having. There's brown, yellow, orange, black, white, grey, green, red in all different shades. It's really easy to stick to gender neutral. Actually, I seem to have ended up with a baby's wardrobe of bright-citrus colours which look really cute and colourful, reminds me of a rainbow.

Not to mention the fact that the baby doesn't actually care what it's wearing or what colour the nursery walls are (although I'd steer clear of blood red walls, just in case).