Avoid the following situation:

• turning on the coffee maker for a mid-afternoon boost
• being asked by one's lovely wife to please do some quick house hardware chore in the garage
• confirming that it needs to be done immediately
• doing it
• returning to kitchen, observing the output coffee cup is much less than fully full
• wondering whether something was wrong with the machine, or the water, or the cup, or something
• noticing that there is another mug nearby
• noticing that it has fresh coffee stains in it
• suspecting the only other adult in the house of coffeenapping
• not confronting her about it
• lamenting instead on the blog

Unfortunately, I don't know how to avoid this situation.