mountain bike switching tyres I get a puncture, why, am i doing something wrong? I have a mountain bike with 2 sets of tyres, a road set and off road set, when I switch between them about 1 in 3 times I seem to get a puncture, and I can't see why?  I've gone though 4 innertubes now doing this.

So I'll inflate the tyre, cycle around for 30-40mins then I'll notice something wrong, and it's almost flat?

Am I doing something wrong?  I deflate the tyre, flick off the tyer and innertube (using a plastic thingy from Evans, so no sharp bits there), put the innertube inside the other tyer, put both back on the wheel and inflate.

Any help would be appreciated, if I don't reply right away I'm probably walking down the cycle shop buying another innertube. :-)
A theory:  There is some damage or debris on the inside of your tire.  A second theory is slightly out of position rim tape.  A third theory is your technique may be leading to a pinch flat.

Questions to help diagnose:
When you switch between tire sets, does the flat happen only going to one set of tires?  Or does it happen no matter which tire?  If it's just one tire set, then inspect the inside carefully for debris or other issues.

Next time the tire is off, look at the rim tape carefully and run a finger along it. It should be smooth and sturdy. You may also want to check that it's secure all the way around. If it is moving around during tire changes it could be exposing a spoke nipple.

The third theory would be that if the tire it tight or tough to get on then perhaps you are pinch flatting during the tire change. Advise here is to go carefully...