I caught one in the kitchen on my Poundland electric fly squatter, and held the killing button down for about 30 seconds. Small sparks, strong smell of burning fly flesh (yum, sanitary), but Mr fly just kept on buzzing.
In an humane attempt, I tried to crush it - but it got away. Probably in great pain, what have I done?
For future executions I recommend fresh batteries. This thing is going to come for me in the afterlife :-(