It happens very often to have, in the same FOV of your target, another asteroid. Sometimes it can be a new one, but almost always it’s a known one. Sometimes it can be very near your target, but almost never it’s BEHIND your target.
That’s what’s happened to me during my last observing night (the 69th from the beginning of the year), between July 20 and 21. I was imaging NEO 2011 NZ, and I’ve noted that a MBA was very very close to it.
After stacking all my 20 images I measure them, with the appropriate motion vector for the both of them. The result was the following:
2011 NZ: 20h 11m 03.47s / -16° 51′ 53.1″
K1419: 20h 11m 03.50s / -16° 51′ 53.1″
If my math is correct, this means only 0.43″ from each other! That’s sounds pretty close!
In reality, the distance between them was 356.3 millions km (2011 NZ was 0.1944 AU from us while K1419 was at 2.5761 AU).
The image is not very good due to the presence of the Moon (71% 60° away), the altitude of the target (27°) and the bad seeing of that night.