What he probably did was to fill it with the bed partially raised. This allowed him to overfill it. Then, when he let the bed down, the extra fluid had nowhere to go, and so it took the path of least resistance, and blew out the seal.
The proper way is to leave out the plug the first time you lower the bed. That way, any surplus fluid can escape. Then raise the bed and replace the plug.
Also, make sure the bed is SECURELY blocked up anytime you are working under it. A more desirable alternative is to get at it from underneath if possible, but that's not always possible.
