For example, Xenon headlights should look like the image on the bottom. If they look like the image on the top, you've installed Xenon bulbs into a reflector designed for Halogen bulbs, and now everybody is blind while also laughing at you for being too cheap to install them properly.

Another example

It's illegal in many places, too. Best case, you'll have to undo the modification to pass inspection. Worst case, you get pulled over and learn your license was suspended 2 years ago and apparently the previous owner packed cocaine into your wheel well.

Please, stick to the halogens your car came with, or install a complete kit with projectors.

Otherwise I will find you.

Edit: By 'Xenon' I mean HID / Arc type bulbs, not Halogen with Xenon gas added. Those are fine. Too blue to be useful, but still fine.