Mine is that I always buy in the sale. I bought dresses from Monsoon a couple of years ago and paid full price as they were for a wedding DD and I were going to. Saw them in the sale a short while later and returned them and bought them again in the sale (we hadn't yet worn them and the labels were still on) as I couldn't bear to "waste" the difference.
My luxury is that I (pre-Covid) eat out quite a bit but even then I make sure I use vouchers/gourmet society.