Susan, this is something I do frequently. I think (for me) it is a brain glitch. Slightly dislexic or some such thing. I switch numbers as well, and have to be careful with balancing my checkbook.

The mind wants to fill in, and if you're tired it can be worse. One of my favorites that has always turned my head is a sign advertising "Martial Arts". I read it as Marital Arts and have to look twice, wondering what the heck a Marital arts place has to offer. I've done that more than once!

Margarita