Is ignorance a good excuse? The answer is "no." That's why you should play it safe by declaring anything new that you bring back with you.
"The biggest threats are travelers who are smuggling drugs, animals, money or pirated computer programs," Mann says. "An important issue today is that there are people who can potentially export software programs or microprocessors on their outbound travels and import them on their way back in. You can store a lot of information on a disk and carry a lot of chips in a carry-on bag. The Customs Service has made enforcing related laws top priority."
Most travelers are "good, honest people," according to Mann. So, for example, if you bought an envelope of fresh spices on the island of Grenada for your mom and shoved it into your purse and forgot to declare it, you'll probably be given a break (although you'll have to give up your spices due to laws aimed at protecting human health from disease and agricultural products from infestation).
On the other hand, U.S. Customs officials aren't nearly so kindly disposed towards people carrying unauthorized firearms or narcotics, and they will certainly detain you. Nobody enjoys being "frisked," but the U.S. Customs and Border Protection has congressional authority to conduct, as it deems appropriate, bodily searches of anyone entering the United States.
Likewise, nobody wants to be the person who's pulled aside and asked to open his luggage for search. How do customs agents decide who to pull aside? In some European airports, there are literally "traffic" lights in customs lines -- red means stop and get checked, and green means go on through. However, U.S. Customs agents make these decisions based on a sort of "sixth sense" they've developed after years of observing many, many travelers, according to Mann. This "sixth sense" kicks in when a traveler appears to be hiding something or attempts to make a false declaration statement.