Changed the winter tyres off the mountain bike, and with some inevitability, one of them (very, very slowly) deflated with the summer tyres on.
Minutely examine the summer tyre to no effect, so assume I'm an idiot and have somehow damaged the tube, find a tiny pinhole in it, patch the hole, leave tyre pumped up overnight before planning to refit this morning just in case.
It then occurs to me whilst lying abed that the puncture may have been caused by something in the winter tyre, so get up this morning, check the tyre is still inflated (it is) then minutely check both winter tyres. BAZINGA! A thorn is discovered and removed. Is there anything more satisfying than the removal of a hard to find sharp from a tyre?
The winter tyres were last pumped up about 2 months ago, and last used two weeks ago, with no discernible loss of pressure. So the thorn pinhole was perfectly self-sealed.