Instructables
Broken zips can be a right pain in the rear end, in this Instructable DoomMeister shows you how to effect a repair to a zip (zipper to all you Americans out there) that has one of its teeth missing.

In a country like Blairstrip 1 you need a good raincoat and DoomMeisters favourite one has a broken zip. Rather that buy a new one as the corporate overlords would have us do, or indeed to fully replace the zip DoomMeister will repair the missing tooth using his awful sewing skills and a piece of a cotton bud (Q-Tip).
 
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Step 1: Reassemble the Zip

If the slider has come away from the teeth of the zip as it will be prone to do if a tooth is missing, reassemble the zip by pushing the slider back on to the teeth making sure to get the tape smoothly and un-kinked between the groove in the slider.

Push the slider as far away as possible from the missing tooth whilst you are working.

Step 2: Fashion An U Piece

In order to stop the slider coming away from the zip again we must replicate the effect that the tooth had in keeping the slider on the tape. To do this the DoomMeister used a small U section cut from the middle of a cotton bud. A small brass clip would be a superior solution

This U piece will not help the zip stay together, hopefully there are already enough teeth for this but will prevent the slider coming away again.

Step 3: Sew the U Piece in Position.

In order to retain the U piece, sew it to the tape. If the U piece is similar to the one the DoomMeister made then you can sew though it although you may need the help of a thimble or some pliers to get the needle through the plastic.

When you have tested the repair to check that it works and the slider does not foul on the repair then you can make it double permanent by covering your new sewing with cyanoacrylate adhesive (superglue).