When puncture protection is no protection

Flat tyres are the curse of my cycling life. There are few things less pleasant than having to stop on the side of a busy road, in bad weather, to change a bike tyre which is covered in who-knows-what-toxic-road-sludge and all because some drunken moron (who will probably run me over in a month's time) threw his empty beer bottle out the car window. At one point I was getting a puncture every couple of weeks until i) I got into the habit of regularly inspecting my bike tyres for sharp objects which were stuck in the rubber but hadn't yet caused a puncture, and ii) got myself some tyre liners from Slime. Slime are the folks who make those fancy tyre tubes filled with some goop that is supposed to be self-sealing if you get a puncture. I'm not sure exactly how the goop works but I imagined that it could glue a tube to a tyre, or leave you with a slow leak that couldn't be patched afterwards of something. The tyre liners, however, are nice and basic --- a thin(-ish) strip of some flexible plasticy stuff. It feels sort of soft if you poke it but it seems to stop things making their way into the tube if they puncture the outer tyre casing. I've had a pretty good run with the slime liners so far, a few thousand kms cycled with no flats --- though I've still had to replace a couple of tyres when they got so cut up that they started to split. But even then, the tube stayed fine --- until last Thursday when I was cycling home down the side of State Highway 2 heading into Wellington. I felt that dreaded squishy feeling coming from my rear wheel. Luckily, I was on the part of the road where it is possible to climb over the barrier on the side of the road and use the track/cesspit/official cycleway to change my tyre, with an extra couple of metres between me and the cars. I couldn't see anything obvious stuck in the outside of my tyre. (I've learned to always look for things after replacing a tube only to have the new tube be punctured by the same piece of glass still stuck in the tyre.) The slime liners still looked fine too; but there was an obvious hole in my tube, running along a thin line. It seems that where the ends of the slime liner overlap the upper-most end had been pressing slightly into the tube (I try to keep my tyres at about 120PSI) and had pressed all the way through. So --- a puncture from my puncture protection. I'll stick with the tyre liners anyway though. I've certainly had far, far fewer punctures since I've been using them. It's just annoying that I don't seem to be able to get free of this problem of flat bike tyres.