Deep sky exposures get photobombed all the time. In fact, 1 out of 30 of my exposures on average of a satellite trail, usually nothing too spectacular. Once in a while, I get photobombed by terrestrial aircraft, and those tend to be a bit more interesting. Here's one from last night, which I found pretty cool because the beacon light underneath the aircraft lit up mid-way through the frame and exposed the engines with just enough light for it to show up quite well in my 5 minute exposure.
Based on my focal length and position in the sky, it appears this aircraft was at cruising altitude, at least 25,000ft. Now if you look at the whole picture, it become quite interesting. A terrestrial transportation aircraft moving through our planet's atmosphere.. with a massive nebula in the background which harvested the heavy elements used in the aircraft's construction.