Distracted?
I get out of bed and notice my throat is aching a little, so when I go shopping I gotta buy some new lemon juice to put in my tea (whenever I'm feeling a bit ill I revert to some hot tea with plenty of lemon juice and honey, combined with just sweating it out. Works quite well).
I still have an empty glass bottle at home that I need to get rid off, so I put it in my coat and head off to the store. I get back home and still have the empty bottle on me and no fresh lemon juice (or lemons) to be seen. Go me. Guess it'll be tea without lemon and just honey :)