Numbers that aren't multiples of 5 on microwave timers, prices, etc. Sometimes I set timers to something like 3:49 just to bug people. I also bite into candy bars sideways out of spite.
Don't most prices end with .99?
Well yes but, from what I've seen, people usually obsess over when numbers are multiples of 5. 3.95 instead of 3.99. 3:00 instead of 3:01.
The $3.95 thing was explained to me as a legacy of old British coinage, where the money was pound notes (or, ages ago, gold coins), shillings worth 1/20 of a pound, and pence at 12 to the shilling. When they decimalized, the old pence were translated to new pence at 100 to the pound and the shillings component of the price was translated into 5 new pence. I'm still a little skeptical, though, because the US has used 100 cents to the dollar since Thomas Jefferson's time and why would we care about British coinage, old or new?