Bottom club Warrenpoint Town made it three wins in a row with a remarkable result away to Glentoran.
Martin Murray, making a big impact on loan from Cliftonville, scored a superb 15th-minute opener, beating his man and curling a superb shot from 22 yards.
Former Glentoran midfielder McGuigan ran clear to make it 2-0 just before half-time.
Murray scored another fine individual goal on 72 minutes and Johnny McMurray got the fourth for the visitors.
Murray ran from deep and beat four Glentoran players before delivering a shot which proved too good for home keeper Elliott Morris.
Then McMurray, also on loan from Cliftonville, stayed onside and went through to complete the scoring.
That was the prompt for disgruntled Glentoran supporters to head for the exits while Warrenpoint supporters were surely rubbing their eyes in disbelief.
Warrenpoint have gone six league matches unbeaten now, having previously lost 13 games in a row.
With Carrick Rangers winning to move out of the bottom two, Warrenpoint are just one point behind Ballinamallard who are now one from bottom in the table.
It was a fourth straight league defeat for Glentoran who are sixth in the table.