“I forgot” sometimes means “it wasn’t important to me.” Be honest about which one it was.

Memory is not neutral. Things that matter to you tend to survive the week, and the ones that slip out were usually competing with something you cared about more. Sometimes it really is a full head and a bad day. Often it is a ranking you would rather not say out loud, and the other person can usually feel the difference.

When you forget something that mattered to someone, ask yourself which it was before you answer them. If it was priority, say so honestly, and put the next one somewhere it cannot slip. An apology with a system in it is worth more than one with a promise.