Start a new game of Classic, get 1 point in Light element and enough gold and build 2 Light towers. On the first waves, creeps should die with two hits, but they don't.
The first hit leaves the creep at 30% health, and the second one at 0% but the creep is still alive! The towers must hit it once more to kill it. I'm pretty sure it happens with Nature towers too but it's harder to notice since they shoot faster.
I also noticed it only happens when both projectiles are shot at the same time towards the same creep. If only 1 tower hits 1 creep, it takes 2 hits to die normally. If both towers are hitting the same creep, with both projectiles flying towards it at the same time, they leave it at 0% hp but it's still alive.
Edit: happens with any tower at any moment of the game. If two or more projectiles are shot and flying towards the same creep at the same time, it won't die and will need a tower to hit it again at 0% health before it dies.