How the hell did I miss these?

I thought I knew, but I didn't...

...then I thought I figured it out, but I wasn't sure...

...then I forgot...

...then I ran into this again...

...then I knew that I figured that I thought that I wasn't sure if I knew or if I forgot...

...then I got a headache...

...so I tried to forget...

...but I couldn't...

...but I'm sure I did...

...then I ran into it again...

...THEN I knew I didn't know in the first place...

...but I do now.

I need to lie down.