In the past, I've always ignored manufacturers' specifications about normal/optimum operating temperatures for electronic goods. But last week, I learned from personal experience why it may not be wise do this with the HD5/HD5H. On a hot day, I was wearing jeans with particularly tight pockets - stuffed into one of which was my HD5H. It had been running for a couple of hours solidly, in playlist mode. Suddenly, it skipped a whole track and went back to playing from near the start of the playlist - completely randomly! Later, it just switched off during normal playback. I switched off the player, put it somewhere cool, then ran the same tracks/playlist right through again, with the unit on my desk at work rather than in my pocket. Everything was fine. Can only assume it was a heat-related hiccup - hot day, plus a human body temp. of over 38 degrees, I guess (upper limit for HD5/HD5H is 35 degrees).