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?