Projectile Speed Example

Demonstrating additive projectile speed reduction.
The Shard upgrade to Arc Warden's Magnetic Field grants it 75% enemy projectile speed reduction.
Faceless Void's innate slows enemy projectiles by 50% around him, as does level 3 Time Zone.
It seems projectile speed reduction has been coded as additive, meaning you can push projectiles back towards their origin. I wouldn't call this a bug, necessarily, but reductions in Dota are usually multiplicative, so it's certainly non-standard.
However, Faceless Void on his own can reach exactly 100% reduction. At that point projectiles play their "on-hit" animation - see how they all vanish at 00:25 in the video. Despite that, the projectile is still hanging in the air frozen, now without a model. Once the speed reduction lowers, the invisible projectiles resume moving toward their target, and all hit at once - see Arc Warden's health plummet at 00:30.