Pics show how to restore  the on/off
function on HF MiniMag light.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=3852
The On/Off button ratcheting  mechanism
in these lights will wear out after some use. 
This mechanism is not repairable. When it
fails, the light stays on constantly and cannot
be turned off with the button. I restored mine by
making a sliding sleeve from 28GA steel sheet,
rolling around with 1/8" overlap, and super gluing
the joint. Now, to turn on, I just slide the sleeve
back so the button pops out. To turn off, slide the
sleeve back over the button. Works good.
JR