Tyre liner causing a slow puncture -- am I doing something stupid On a beater bike I've got (25 year old Raleigh MTB) I run tyre liners because I had spare tyres and was too cheap to buy good anti-puncture ones.  They do their job quite well -- I've just picked broken glass out that had gone through the tread but stopped at the liner.

But the reason I found the broken glass was that I had a flat.  I knew the tyre was a bit soft but over the weekend it went right down.  On inspection I saw this on the outside of the tube:



That curve matches the end of the liner, and with enough air in the tube I could feel it leaking.

The liner doesn't feel like it has a rough edge (we'll find out because I left the liner in with a new tube), and it's been running fine for over a year.  This would appear to defeat the point of liners, so am I missing something?  Does too high pressure cause this? Too low?
I've never seen this exact flat, but I can see how it can happen.  It's why some brands of liners now have rubber tips at the ends of them.  a little filing just to be sure it's smooth there with an emery board should take care of it.  Or as suggested in the comments, a piece of electrical tape should help.