A few tidbits of information may be helpful.
What load were you using? (Powder,bullet,primer.)
What kind of loading? (Progressive, turret, or single stage.)
The powder, and charging method would be of the most importance.
If single stage how are you charging? (Powder measure, or dipper)
If progressive, or semi progressive such as a Lee turret with an auto disc it could have been from a low hopper, short stroke of the ram, or bridging of powder. If so be careful. If it causes one round to be under charged. Then it will cause another to be over charged from the bridge breaking.
(Bridging is when a flaky or bulky powder sticks in the throat of a powder funnel, or measure causing an under charge. When enough weight, or the jarring of the press breaks it loose it over charges the next one.)
Inattention, and being in a hurry, or being distracted that brief moment can lead to one forgetting that a case was not charged.
The good part is you caught the event, and know what to watch for in the future.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.