In one of my games, after I had three gateways, I realized that making zealots wasn't costing me any minerals. This persisted for about 3 minutes then everything went back to normal. I believe I had selected every gateway at the time. I'm sorry I can't be more specific than that.

If this was a team game in which one or more of your teammates left the game, the resources they had are used by you before it pulls from your resources.