Given that the one arm dumbbell bench press is a reasonably common exercise that puts all of the load on one arm and yet doesn't seem to have any elevated risk associated with it, it seems extremely unlikely that accidentally bench pressing with an unevenly loaded bar would be any more likely to cause injury than a correctly loaded barbell bench press.
Furthermore, there just really isn't any plausible mechanism through which an unevenly loaded bar could cause injury, unless it was so unevenly loaded that this caused you to drop it and it landed on someone. (This is more likely in a squat, where the bar is supported closer to the middle.) If you had 15kg on the left side and 12.5kg on the right, the only expected consequence of that is that your left arm and shoulder will tire out as quickly as they would if you had 15kg on both sides, while your right arm and shoulder will not fatigue as quickly. So with 15kg on one side and 12.5kg on the other, you would expect to be able to do as many reps as if you had 15kg on both sides.