Nothing, as long as it's a low voltage capacitor. Do it to a filter cap in a power supply and you'll have a huge spark and probably damage your screwdriver. Best practice if you're working with large capacitors is to discharge it (safely, using the tool I described), and confirm it worked using a voltmeter.
I learned dealing with 600V monster filter capacitors in vacuum tube power supplies, they'll happily vaporize a screwdriver and knock you off your feet in the process.
I learned dealing with 600V monster filter capacitors in vacuum tube power supplies, they'll happily vaporize a screwdriver and knock you off your feet in the process.