You could trash your harddrive by forcing it to go too fast with hdparm. Very few people ever touch hdparm, and those that do are experts who know what they are doing. The point is that it is there if you need it. If I hadn't told you about it, you may have never heard of it. The analog of this would be overclocking your CPU: you can cook it if you aren't careful.
Older versions of XF86 could wreck old (say 1993 or before) monitors if you FORCED X to use too fast refresh rates. What would happen in such a case would be the monitor picture would be scrambled, and you would need to turn it off and reconfigure X. Modern monitors have safety circuits that cut the picture if too high a refresh rate is supplied. Furthermore, modern XF86 autoprobes multisync monitors (such as the 6 year old monitor I'm using right now) for refresh rates within a range you specify at setup. Hence, this is not an issue anymore, AFAIK.
Do not be afraid of linux killing your hardware.
[ August 26, 2003: Message edited by: M. O'Brien ]