OnTriggerEnter only sent to one of the colliding objects, not both?

So I have a player, with a SphereCollider trigger and a Rigidbody. I also have a pants-load of bullets, each with a SphereCollider trigger and a Rigidbody. The player is the first thing spawned into the scene (which may be important in a minute).

When I start playback, the player can collide with every bullet in the way you'd expect. After the player dies and respawns once -- which means that GameObject was disabled via SetActiveRecursively, then re-enabled in the same fashion three seconds later -- his collision with bullets becomes inconsistent, in a very peculiar way: some bullets, seemingly at random, are now completely un-collidable no matter what, while the rest of the bullets continue to collide normally.

The script logic that's telling me a collision has occurred is located in the Player's OnTriggerEnter. In a fit of desperate flailing, I tried duplicating that logic into the bullet's OnTriggerEnter... and this fixed the issue entirely. O_O

So what it looks like, is that for some as-yet-unknown reason, I'm getting into a state where some bullet->player collisions are only calling OnTriggerEnter on the bullet, and not on the player, while other bullet->player collisions are calling OnTriggerEnter on both objects.

Uh... WTF? Isn't this, like, impossible? O_o