What happens if you are caught (accidentally) not declaring something at customs?

It happens regularly in NZ, because of the fruit and plant matter laws.

The point is, you're reminded so many times, and given a form where you have to tick 'I don't have this, or this, or this', that not knowing is really not an excuse. Like any country, ignorance of the law is not a valid reason to break it. By the time you've got to that point, it's assumed you can't be ignorant and are not declaring it on purpose.

As a result, there's a good chance you'll be fined.

Sometimes if they can see it's an honest mistake - eg you've taken five apples out and forgot the banana under three shirts at the bottom of your bag, you might get a warning. Not cleaning your hiking boots will get you a talking to, but they'll often take them off you and clean them for you anyway.

Naturally if it's a bigger problem (drugs, weapons) then there's potential for arrest or jail time, but for smaller things, it's usually a fine. For example, even famous actresses can get fined for forgetting to declare fruit.

On the upside, they sometimes have fruit sniffing dogs pre-declaration, looking for stuff, and if they find an apple or orange at that point, they'll just ask you to throw it away. As far as I've seen, it's only when you've officially signed a form declaring you have nothing, that you are now breaking the law.

Depends on the country, on the actual thing that was found (i.e. which law it violates - agricultural, alcohol&tobacco, customs duties, etc), on whether or not it's actually illegal to import (as opposed to just subject to declaration but legal to import), and I'm guessing on the mood of the customs official as well.

Supposedly, in Canada an undeclared food item (even a single apple), if it's actually judged to be illegal to import when they do find it, can result in close to $400 in fines and a mark on your record that makes future searches much more likely (and of course you can probably forget about trusted traveler programs like NEXUS). I haven't personally verified this information though.

Customs agent manuals and relevant rules for Canada (and I'd imagine for the US as well) can generally be found online (through Freedom of Information requests or through direct publishing by the relevant agencies), so with some effort it should be possible to find an exact answer if you aren't afraid of legalese.

It happened to us once long ago, entering the US. The culprit was a seed pod we had no idea was in our suitcase--we had packed in the shade of a tree and apparently it dropped a seed pod in. It was right on top, not hidden in the slightest. The fact that the bag packer was my mother--who was totally blind--probably helped in them deciding we had no knowledge of the wrongdoing. No fine.

Two times it has happened to me. Maybe 15 years ago bringing Cuban cigars over the bridge from Canada to Detroit. I just had to destroy them.

The second time was two years ago bringing milk from Japan into Orlando International. They confiscated it.

No large quantities and nothing highly dangerous. No fines.

It is similar to traffic violations. You are eligible for a penalty, but it is up to the customs officer to choose between a penalty or a warning.

I know various examples for both cases. Ranging from custom officers simply ignoring booze which was not even hidden, to getting a ticket for a half eaten apple simply forgotten to throw away because of putting passport, tickets and money back in place at immigration.

In the end it is up to the custom officers interpretation of your intentions or his grumpy mood.

I once crossed the Swiss border by land with a fishing knife in the car I was driving (I didn't know it, it had been there for a long time, left by another family member who had used the car). That's the only thing that was found after a 30 min search.

The border guard took it and went to his desk, came back a few minutes later and claimed that technically (“streng rechtlich”) it was a weapon and we would be liable for a CHF 400 fine (or something of that order). He nonetheless decided to let us in without a fine and did not even confiscate the knife IIRC.

It's an anecdote of course but it shows that border guards have a lot of leeway, even for things other than food or regular goods slightly over the allowance.

In Israeli customs, you would be required to pay a fine (equal to the duty) regardless of whether you take the item. If you wish to take the item, you'd have to pay the duty too (so effectively you'd be paying double duty).

For example, for a 400 ILS bottle of Whiskey, you'd pay a 411 ILS fine. If you choose to take the bottle, you'd also pay a 411 ILS duty, summarily 882 ILS (in addition to the purchase price).

Legally, failing to declare is an offense, and they may press charges for serious contraband, but common offenders (say, an extra bottle of liquor) just get the fine. From personal experience, getting pulled over at the green corridor in Ben Gurion Airport is rare (never happened to me), but your mileage may vary.

Source: http://www.ynet.co.il/articles/0,7340,L-3093102,00.html

Chile is pretty serious about controlling fruit, seeds, and nuts being brought into the country. A few years back, someone on a trip with me brought along gorp (trail mix) on the advice of the tour organizer. I'm not sure how it was detected, but it was confiscated and they were fined for having it. I don't recall the exact amount of the fine, but it was in excess of $100.