This happens to me around once every month or two. The problem is that I usually take it in the morning right when I get up so I'm still sort of on autopilot. Any activity that I do every day as part of a routine can simply not get committed to memory if I'm thinking about something else.

Usually I just have to think really hard about it or (more likely) wait an hour and see if the morning brain-fog lifts. I don't want to accidentally double up for obvious reasons, but in the worst case, I just skip the AM dose to be sure and take the noon dose a little earlier if it becomes obvious I missed it in the morning.

Nothing like trying to remember if you took your medication to improve poor focus and attention, right? ;)