Planning a vacation? Start by unpacking.

Shifting rules and over-zealous staffers can trip up even the most checkpoint-savvy traveler. (Flip-flops on or off? Is mascara a liquid or a gel?)

But it may not be the TSA you have to worry about; watch out for other travelers.

So far in 2008, the TSA has discovered 519 guns – many of them loaded – in carry-on bags at security checkpoints. That figure doesn’t take into account weapons found over the past few weeks. According to the TSA Web site, 29 firearms were found two weeks ago, and 23 were discovered last week.

The most common explanation from travelers? “Oops, I forgot that was in there.”

Find out what else shows up at airport checkpoints and learn about the new rules for “checkpoint friendly” laptop bags in my Well-Mannered Traveler column posted today on MSNBC.com.

(Column illustration by MSNBC.com’s Duane Hoffman)