Nothing kills a red carpet look quicker than bad hair. Whether it's too casual or too contrived for the dress, the wrong hairdo can make or break a star's look.
For more casual special events -- say the Grammys vs. the Academy Awards -- celebrities often go with slicked-back ponytails or messy buns for a more laid-back look even when paired with a formal gown.
One of the worst mistakes you can make for a special event is to go with your everyday hair, unless your hair is fabulous everyday (Halle Berry comes to mind).
Even if you only add a sparkly barrette or a pull back part of your hair, it's enough to set off the look as being special.
In fact, just going with a fancier version of your everyday look is much easier to carry off than going with a complicated beehive or some other look that's hard to pull off.