When you tell your wife you'll be 45 minutes to an hour late picking her up from work because you have to get to the LHBS before they close so you can brew on your day off tomorrow.
Or
When you buy hops specifically for a hopped mead, but need to brew a beer too so the rest of the hops don't go to waste.
Or
When you buy hops specifically for a hopped mead, but need to brew a beer too so the rest of the hops don't go to waste.