There are multiple occasions this can occur. The two most known ones are:
1. When jumping out of a vehicle, if the game isn't given time to load the rubber bullet script before the player begins firing then his tracers will be lethal.
2. Rubber Bullets work by dealing damage and then (within milliseconds) healing the target player to 100% health. If the target player is low enough health then he will be killed by the initial damage (usually a headshot does this) before the script has time to heal him to 100%.