I remember a fair few years ago using big two handed grinder to clean a weld up whilst i was in the garage. After a short while I could start to smell smoke and hear slight crackling noise. Turned round to find a big paper bag full of a fine wire wool was glowing merrily away and burning away the bag it was in. The sparks from the grinder had landed in the wire wool and set it off.
Luckily this was only minor and easily sorted. My worse fire was when my gas fluxer attached to my oxy-acetylene kit failed big time and fireballed me and the garage.
Colin