It is absolutely essential to carry a hot-melt glue gun in your bag. When your battery suddenly dies (and you know you'll never be able to find another) and the only bass available is a fretless (and you don't play fretless) from some stranger that wanted to sit in with your band for a few songs, run to the bar/restaurant/dive kitchen, grab some toothpicks and heat up that glue gun. If there are fret lines, just glue those toothpicks near them. If there are no fret lines, use your best judgment. Everything will be just fine.