Our military has some of the best pilots in the world, and they have checklists for everything. There are times that even they miss things... No doubt most don't have problems backfeeding a panel, but the potential is there for a serious issue.
Even with the appropriate panel installed, there is potential for a serious issue if it is not done correctly. After all, the electrician is following a mental checklist of his own when he installs it. But as long as you turn off the main breaker before you do anything, that'll eliminate the most serious issue in backfeeding.
Is backfeeding against code? Sure. So are most of the Christmas lights I see people put up. You're not going to find an inspector in all of New Jersey right now to cite you with a backfeeding violation, and one that is temporary at that. Note that in most states it's against code to wire your furnace so it plugs into an outlet, even if the outlet is on its own dedicated circuit. There are several how-to's on the Internet on how to do this very thing so you can plug it directly into your portable generator. Yet I think over half the houses in the U.S. have their furnaces wired like that.