I had a tough day cycling the other day and couldn't figure out why for a while. Now I know.
I have Sole inserts, which I like a lot. A few months after I bought them I received an offer for two pair of their socks -- for free. I took them up on it, and the socks seemed very nice. I wore them and enjoyed them.
After my rough day, when I took them off, I noticed they were marked, on the inside, "left" and "right."
Now, I can't see or tell the difference, but apparently my body subliminally knew I was riding with them on the wrong feet! Clearly that was the problem. That's a mistake I'll be sure to never make again.