Every time the pneumatic limb is used for special actions (not ordinary like handling objects or running without a rush etc.) roll 2d6. If the result is "snake eyes" (two 1's) roll the chart below:
1. It explodes and is lost. 1d6 damage
2. It breaks down and cannot be used before replacing or repairing it. (cost 100gp)
3. It doesn't respond or work for 2d6 turns
4. It acts stiffly. What ever character was doing doesn't success.
5. Minor malfunction. Results 1-1 and 6-6 calls for this chart until repaired (50gp).
6. POWER BURST! Double effect or other appropriate bonus.
I got this idea from Google+ where +Zak Smith wrote:
Dear Diary: Went in with one hit point, left with one hit point, 250 gp, a magic arrow, a serrated gnoll sword and one leg.
+jason gielbasa talked about pneumatic limb and I just had to blog it!
Will work great with: