The % symbol is an undocumented command on the TI-83 series calculators starting with OS version 1.15. It's useful as a shortcut for percents - it divides by 100, so it will convert numbers to percentages. For example, 50% will become 50/100 or 1/2, which is just what 50% should be.
Although this trick can save you a few bytes, it also makes your program incompatible with old OS versions -- it's up to you to decide if the tradeoff is worth it.
- ERR:INVALID is thrown on older operating system versions.