What alot of users don't know is a psu can get pretty darn hot. The movement after shutting down can allow a weaken component to flex and/or move abit and break its electrical contact and/or soldered area. Which means it was poor to begin with or it was really under a lot of stress, heat build-up. Also componets can crack with similar results. The 300W pus depending on its manufacturer were prone to failure until better wattage units came out. There was a rash of bad psu's for awhile but that was years ago or when the 300W was the norm for many a new system. Designs have changed but low wattage psu tend to be stressed and/or come to failure in short order vs. newer high wattage psu that can get into the 1KW range. Some psu do have a build-in "power fail" circuit which causes the psu to remain-OFF when it fails or reaches certain damage. It self-detructs in order not to be a source of fire and yet the fuse remains intact, go figure. Open the failed PSU and check it out. It may even power-UP if re-installed if one of the above was the cause now that it has cooled(it happens).

tada -----Willy Happy