A non-American friend recently had to file an ETA to travel to Canada. He had a traffic ticket in Texas about ten years ago for crossing a solid white line on a highway. He paid the fine online. Because of the ticket and because he's unusually honest, he answered the Canadian ETA question "have you ever been arrested for, been charged with or convicted of any criminal offence in any country/territory" positively.
This then ended up in a Kafkaesque situation where Canada blocked the ETA and wanted not only an FBI report but also the full disposition of the case. But the Texas court website had no record of the ticket - even when searched by name or case number. My friend was able to get the FBI report which was also blank in terms of any convictions. Submitting the FBI report with a note that the Texas court website had no records resulted in the ETA being approved, but it was a stressful two weeks for this person.
My friend is unsure what to do in the future when asked this by immigration departments (not just Canada). If there's no record of a traffic ticket, should they answer in the future "yes" or "no" when asked about criminal convictions? It's very hard to prove a negative - so if Canada had insisted on seeing the disposition from the court (and not just the FBI report), he's not sure what he would have done. Our American friends are insisting that in the future, he just answer 'no' for the convictions, but that seems wrong too in light of other people suggesting that hiding things from immigration is much worse than just being open about it.