Perhaps one of the most common mistakes we see is the word everyday used when it should be two words: every day. Examples of correct use: Customers use us every day to find mistakes in proposals and slide shows. Misuse of the space character is an everyday occurrence for some of our needier clients.
Other words posing space issues include abase, blackout, pickup, apiece, altogether, runoff, and blowout.
An interesting one is goldbrick. With the space, a gold brick is worth a lot of money (around $1449 per ounce as of 4/5/2011!). Without the space, a goldbrick is something that appears to be valuable but is actually worthless, or it is a person who shirks assigned work.
Now for current events: The word bail has at least four meanings.
When combined with out, it can take on two new meanings. With a space, bail out is a verb meaning to parachute from an airplane or to abandon a harmful or difficult situation. Without the space, bailout is a noun that means "a rescue from financial distress."
So...if you are writing about related events these days, use the one-word form, as in "The government bailout plan began as the Emergency Economic Stabilization Act of 2008."