Surefire G2 Nitrolon Tailcap spring came off

I wasn't able to get my spring back in so I suggest that you contact SF customer services and they'll send you a replacement.
 
You should be able to put the spring back in by taking a pair of needle nose pliers, grab the spring on the small end, push it into the tail cap, and twist the spring clockwise. Twisting it clockwise while pushing it in should coil up the spring just enough to make it past the opening and into its seat. That's how I've done it on a few of my SF tailcaps.
 
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