Question: I recently got my US visa and I am really happy about it. Also, I recently saw a documentary where a US immigration officer was denying entry to some people. I never thought your entry could be denied if your visa was valid.

What are common reasons for US immigration officers to deny entry in the country?

Answer: 
Most people are refused entry if their intent to enter the country is different than what that visa is for. There are many other reasons as well.
A visa is not a right to enter a country. Citizenship is that.

When you enter a country, two things happen.

* An **immigration** officer speaks to you as you enter.
* The **customs** service looks at your stuff.

Those people have access to all your data in the computer, so they know all about your visa application. **If the conversation or your stuff suggests an intent contrary to law or your visa conditions**, that could lead to a refusal. For instance, on a UK border-patrol TV show, someone's luggage was found to be full of business clothing and 20 copies of the person's CV. That was *prima-facie* evidence of "seeking employment", not allowed on their visa or [VWP](https://en.wikipedia.org/wiki/Visa_Waiver_Program), so back they went.

So it is possible to have a valid visa, and then "put your foot in your mouth" at the entry interview, or be seen bringing in something foolish. For instance, it's a lousy time to try to smuggle cigarettes - people still do that, and still get turned away. (Trust me: they have cigarettes in stores here.) What makes it smuggling is "not declaring it" - declaring it greatly improves consequences for honest ordinary mistakes.

Because America is a *federal* country, this can blind-side people since we have a cacophony of differing government laws. Many states are simpatico with marijuana use, but it's a Class 1 narcotic to the feds. So drug paraphernalia or the smell of pot will doom you, even if it's legal in the state you are landing. (This is *especially* a problem at the Canada border.)

Speaking of that, border guards can prohibit things that are legal in both places - Canada won't *let in* marijuana and paraphernalia, even though it's legal in Washington state, and legal federally in Canada. A laptop full of porn is a problem here.

Note that they can also inspect your phone and your social media activity both for contrary-to-visa activities and simply being undesirable... and if you say "well, I won't give them my passcode!" they will simply refuse you entry. So data hygiene is a good plan... also, not being a jackass on social media they know about.