I’ve got a nasty habit when stripping extra line from my reel for longer casts: I pull my line down and away to get more out. Depending on the style of you reel you might end up with some pretty serious channels being dug in to your reel like the ones above over time. I think this mainly happens to me after a long day of stripping streamers where line distances vary quite a bit. After a while these grooves start to damage and actually cut your fly line.
From what I can tell the dirt and grime that collects on your line can actually start wearing into the metal. Granted I’m lousy at taking care of my lines, but I think this could happen to anyone if you’re constantly stripping against the metal. I’ve had it happen to two different trout reels over time.
In addition to regularly cleaning your line, one easy solution—and it takes a bit of extra attention while fishing, is to strip up and out rather than down and away.