Championship (second division) Coventry were leading 1-0 and on the verge of dumping the 2008 winners out of the cup when captain Stephen Wright threw the visitors a lifeline with a 90th-minute own goal.
With penalties looming after a deadlocked extra-time, Mokoena headed home to earn his side a 2-1 win and revive hopes of another lucrative Cup run for the Premier League's bottom club who next play Sunderland. Birmingham City ended Nottingham Forest's 18-match unbeaten run with a 1-0 win. Barry Ferguson broke the deadlock against the run of play with a left-footed shot in the 62nd minute.
Sheffield United earned a ticket to Premier League Bolton Wanderers after beating Championship rivals Queens Park Rangers 3-2.
Championship Derby County overcame League One (third division) Millwall 5-3 on penalties after their match ended 1-1 after extra time.
Cardiff and Bristol City must replay their all-Championship third-round tie after drawing 1-1 at a snowy Ashton Gate.
