This seems an obvious one in as much as it wears our sails out however it is often the wear we don’t notice that does the most harm. We all notice a hole forming where the sail hits a spreader or radar and get it re-inforced but it is the more inconsequential wear that needs the attention. Broken stitching on clew patches, U.V strips and seams of sails can lead to the untimely demise of your pride and joy. Though I hate to admit it my Gran was right, a stitch in time really does save nine (she would be so pleased to know I took it in). The other place to keep an eye on is batten pockets and under luff slides on mainsails.
Batten pocket wear can lead to some catastrophic failures and some expensive broken battens.
Luff hardwear can often hide some nasty wear that leads to the luff rope failing and ultimately the luff of the sail breaking and an untimely return to the dock.