You have to get a temporary import/export permit and country-specific insurance every time you cross a border. I ended up crossing 33, so that's plenty of paperwork to potentially get fouled up on. And in some countries checkpoint cops love busting gringos for their paperwork, because if they're missing something mandatory, that's easy money.
I screwed up the US/Mexico border first by not having done my research. Crossing from Presidio to Ojinaga, I made sure to get Mexican insurance, but didn't think to set Banamex bond for the temporary import permit — not required in the border zone, so the customs officials don't mention it. I didn't have it, and if any official in the length of Mexico had asked me for my paperwork (despite passing multiple checkpoints), that would have been full legal grounds to seize the bike. But I made it to the Guatemalan border trouble-free, and the customs exit officer nailed me for it there. "It's easier if you just pay me the fine." Around $50, I think. I got change from his wallet.
The second not doing my research screwup was not bringing my original title document to the motorcycle with me, just some color photocopies. In most cases, copies are fine — after all, we don't drive around with our titles in our glove boxes. But leaving El Salvador, at the Honduras border, a cop in front of the immigration building decided to make an issue of it. "I'm sorry, but without the original title document, there's nothing that can be done." After about half an hour, with some help from a roadside border-crossing fixer (there are many on the El Salvador / Honduras border, because these situations are notorious there) and a little over $200, the bike and I were through.
Of course, the first thing I did after arriving in Choluteca, Honduras, was to head over to the police station and file a false report, saying that I had left the original title in the top bag of the motorcycle and someone had stolen it, along with a few other miscellaneous documents. When getting yourself out of paperwork trouble, an officially stamped police report is as good as gold. From that point on, at any border where the photocopied title wasn't good enough (Peru, in particular, gave me a really hard time about it), I could show them the police report, show them that the VIN on the report matched the VIN on the photocopied title matched the VIN on the bike, and we were good to go.
Like I said, not NYT-standards behavior.