Thanks so much, everyone!
Rob, stuff like that happens all of the time. And there have certainly been instances in which I've coughed or been knocked into while trying to make a very precise mark. But I'm always able to cover it up, whether I'm just scraping the marks off and redoing the section, or just letting it dry and painting over it. Luckily, oil paint really does lend itself nicely to both of those solutions, especially in regards to scraping, which makes for some awesome textures to work on top of.
Graig