The Health and Safety Executive (HSE) prosecuted the firm following the incident on 19 October 2009 at the company's site at Hartlebury, near Kidderminster.
Kidderminster Magistrates' Court heard how the 48-year-old man, who does not want to be named, was rectifying a fault which had stopped a brick packaging line from moving.
He entered the safety zone around the line, which was protected by a perimeter fence with interlocked access gates, and climbed up onto the conveyor belt to fix the problem. However, the line suddenly restarted, activating a pallet lifter which came down and crushed him.
He suffered a serious puncture wound to his back, cracked several ribs and sustained extensive bruising to his face and leg. He was off work for six weeks.
HSE's investigation found an interlock on one of the gates, which should have isolated the machinery, had been disconnected some days before the incident to allow a printer within the perimeter fence to be serviced.
The company had also failed to carry out an adequate assessment of the risks arising from working on the printer within the perimeter security fence and the controls required.
Speaking after the hearing, HSE inspector Chris Gregory said: "A man suffered potentially life-changing injuries following an incident that should never have happened.
"Deliberately bypassed interlocking devices are a common cause of injuries in the manufacturing sector.
"Free guidance on guarding standards for machinery has been available from HSE for many years and it is unacceptable to see such basic failings, particularly from a large company."
Wienerberger pleaded guilty today to breaching Regulation 3(1) of the Management of Health and Safety at Work Regulations 1999 and Regulation 11(3) of the Provision and Use of Work Equipment Regulations 1998. The company was fined a total of £20,000 and ordered to pay full costs of £11,611.