I'm the type of (cheap) person who only keeps two pairs of shoes at a time: One for everyday use, and another for weddings and funerals. So for me, my impulse purchase was a pair of cute shoes I walked past in a department store one time. My current shoes were working fine and the new ones were really expensive, but they were just too awesome to pass up. So, even though I was already worrying about how I was going to justify this as a necessary purchase, I bought them.
Then, the very next morning, I woke up to find my old shoes half-eaten by my mom's dog, making the new pair of shoes the only pair of casual shoes I now had. Talk about timing!