Mansfield overcame their off-field turmoil to hold on for a replay against first division opponents.
The Stags, with Carlton Palmer in temporary charge in place of the suspended Keith Curle, also had to shrug off a red card for striker Derek Asamoah nine minutes from time.
But despite a frantic finish with Sam Stockley's long range shot well held by Kevin Pilkington and Craig Fagan firing narrowly over at close range, Colchester failed to find a winner.
Crisis club Mansfield, also missing fulll backs Scott McNiven and Craig Woodman, gave a debut to loan signing Fraser McLachlan brought in by Palmer just 24 hours earlier.
Colchester, with defender Greg Halford still leading the attack, made the early running in a low key start with the tall striker twice threatening from long throw ins.
However, after their hesitant start the Stags, without a win in five games, went on to dominate the first half and should have won a 15th minute penalty when Stockley clearly handled a Wayne Corden shot.
United snatched a 25th minute lead against the run of play when Gavin Johnson flicked on a John White cross for Halford to head in his fourth goal of the season.
The lead lasted just four minutes though with new captain Alex Baptiste ending Mansfield's 343 minute goal drought with a stunning 30-yard dipping volley.
Asamoah was denied by a smart save from Dean Gerken and at the other end Mansfield full-back Jake Buxton's last ditch lunge block a goalbound Joe Keith shot.
After a dreadful half hour the tie came back to life 14 minutes from time when McLachlan struck the post after a fine run by Asamoah.
Just five minutes later the striker then became the villain when he was sent off for a two-footed lunge on John White, but Colchester failed to cash in.