Ever put the milk in the cupboard and the cereal in the refrigerator? Or lose your house keys? My favorite is the missing sock after you swore you put the whole pair in the wash? Sure. We've all had inglorious brainfarts.
These can even happen in your writing.
You set the scene: The sky is a brilliant burnt orange, lit by the setting sun over calm waters. The only sound is a lone sea gull crying once, twice, before fading into the distance. The salty air mingles with your hero's musky cologne. He cups your heroine's face. The anticipation of the kiss plays with her mind. She recalls previous kisses with her true love. Her lips feel the sweet haunting pressure as he bends down. She steals a glance at his face before closing her eyes and surrendering to him. Looking into his...his...Shit! What color are his eyes!?!
Don't let this happen to you. Or if it does don't let it throw you out of the writing mindset. Prepare yourself for err duh moments and they will never throw off your flow. I don't know about you but if I were working a scene like the example above and wasn't prepared it would take a little extra something to get back the mood. Instead of backtracking through 15 chapters to his description make a chart or note cards of every character, no matter how small the part they play.
In Birth of a Vixen Father Khel is rarely seen, but in Virus Within he is a juggernaut. I don't want to stop writing one book to reread another in order to remind my self of his eye color or where a scar is located. I have a chart saved on my laptop and a flashdrive. This way I have access wherever I am when the light bulb turns on.
If you don't outline your books you can still do this simple step. When an unexpected character shows up finish the scene then add him or her to your roaster.
I recommend doing this for locations, too. The world is as much a character as your hero or villian.
How do you prepare for potential brainfarts?