A thorn I picked up that only caused a very slow puncture, I left it in for ages and only sorted it out when it got to pumping the tyre up about once a week. Another one was a tiny piece of orange indicator glass, on overtime I came out in the dark to a flat and fixed it by the light in the compressor house but I couldn't find the cause, tyre a bit soggy the next day so I put in some air and pedalled to work , by dinner time it was flat, I repaired it during dinner but it took about twenty minutes to find the glass.