Why do people claim they crossed over to Canada from the US border by mistake?

Many tourists make the mistake of crossing the border into Canada when visiting Niagara Falls. A recent Reddit post on a person claiming that a friend, a Palestinian refugee, went inside Canada from the US border and then got detained while coming back has triggered a major debate on such claims. This particular incident goes like this: the Redditor said a friend and the friend's boyfriend wanted to see the US-Canada border in Detroit. They ended up at a spot where they could not turn around and had to cross the border.The guards allowed them to enter and assured them that they would be able to turn back. But border police stopped them when they were entering the US. The boyfriend was a Palestinian refugee with a valid passport and an expired visa. But he had all the paperwork that he filed for the renewal of his visa. The girlfriend was an American citizen and she was released but the boyfriend was detained -- the friend posted on Reddit.Many Redditors who claimed to be US citizens commented that it is a pretty common mistake and it happens all the time at several other places too but they faced no problem as they are citizens. But many claimed that if the story was true, the Pale...