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[rfranklin]

1. Hoist by their own petard...

A phrase especially appropriate for the Republican bomb throwers--
Meaning

Injured by the device that you intended to use to injure others.

Origin

The phrase 'hoist with one's own petar[d]' is often cited as 'hoist by one's own petar[d]'. The two forms mean the same, although the former is strictly a more accurate version of the original source. A petard is, or rather was, as they have long since fallen out of use, a small engine of war used to blow breaches in gates or walls. They were originally metallic and bell-shaped but later cubical wooden boxes. Whatever the shape, the significant feature was that they were full of gunpowder - basically what we would now call a bomb.

The device was used by the military forces of all the major European fighting nations by the 16th century. In French and English - petar or petard, and in Spanish and Italian - petardo.

http://www.phrases.org.uk/meanings/hoist%20by%20your%20own%20petard.html