This has happened to me twice now. In both cases, I felt so immensely guilty at first, then I got over it because I realized I was tricked. That's right, tricked! I was misled into making the cardinal baby mistake... calling a child by the wrong gender.
Both times it's been adorable little girls. Beautiful little girls whom I referred to as "handsome guys." Both times, those adorable little girls were dressed in boyish clothes. Dark blue with thick, heavy brown sandals. Dark red or bright orange. No bows, no ribbons.
Look, I get it. We all want to dress our kids however we like without confining them to gender stereotypes. "I shouldn't have to dress my daughter in head-to-toe pink!" you say. I totally agree, but you're putting the rest of us between a rock and a hard place.
See, every mom gets ticked off when their kid is mistaken for the opposite gender. What you always hear is the angst about how a little girl covered in pretty pastels is called handsome, or how a boy dressed in solid blue gets called a gorgeous little girl. What irks people about this is the way these people have ignored the clear gender cues from the kiddo's clothing. So the rest of the world hears these complaints and says "Right, this is important. Go by the clothes to avoid ticking moms off." So that's what we do.
Ergo, if you dress your kid in clothing that is more suited to the opposite gender, or that is decidedly neutral, then be prepared for your child to be misidentified. The rest of us have only this to go on, dangit. We're so scared of messing it up, so we stick to the one clue that we figure is foolproof.
If you give us a red herring, you have only yourself to blame.