Let's share how we avoid such mistakes then....
Like the guy above me and skip one and the one above that (oh, wait, that's me... I'm not the chosen one either), I've made such mistakes and ruined a whole bottle of my chemistry before. Since then, I use a system to stop myself from doing it again.
My rule is simple. NEVER have more than one bottle open at any given time. So as I'm done with a step, I pour the chemical from the tank to a cup, then it goes into the open bottle. Pretty simple but effective. Also, as I line up my bottles in the order I need to use it, and place a measuring cup right in front of it. So there is never a confusion what's in each cup. Chems travel vertically from a cup to a bottle.
It's pretty easy to make careless mistakes. I try to avoid it by doing the above and making it a habit.