I have this happen every now and then. It's usually with certain groups of numbers, but I have mild dyslexia. If a 6 or 9 appears inside a group of numbers I will flip it into the other. But upon seeing it a second time it looks correct. sometimes I will write my numbers upside down. I figure it's neurological and not the health of your eyes failing. I've never experienced entire words or signs however. Was there anything different about your day? Extra cup of coffee? Three hours less of sleep?