I lost a map because my cannon was hit by a spore *IN FLIGHT*. It was far enough away from my base to not be covered by my defenses, it was close enough that the creeper pile (either 800 or 2000, I'm not sure which tower hit it) was more than my defenses could deal with. This was in the void and due to the nature of the map I had very minimal defenses.
I've noticed that behavior as well, and used to to my advantage on some maps. (#1200 "2nd")
It's not a bug, it's a feature.