I know as an electrician, that you can have bad problems if you have a fixture that is rated for say a 60 watt maximum bulb, and you use a 100 watt bulb.
I just recently had to troubleshoot a ladys house whose master bedroom breaker kept tripping. Discovered it happened whenever she turned on her closet light, so investigated the switch, all clear there, took down the first light, nothing unusual, moved on to light fixture number 2 and found all the elctrical melted, the sheet rock scorched and cracked, and all the paint an insulation in the fixtured burned out. She had replaced the bulb a few months prior with a 100 watt bulb in a 65 watt maximum fixture
So long story short, do not use a bulb with a wattage rated higher then the fixture you have, the bulb will pull more power then the wire is capable of safely carrying.