I was reinstalling my stem (an expensive Enve one), double checked the torque, and did up the top bolt. I assumed the lower bolts were the same torque, but it turns out they are half as much, and I snapped off the ti bolt below the surface (ie so I couldn’t get to any exposed bolt). Not a case of rounding the bolt (in which case I could have used @Dave_Rome’s piece on recovering stripped bolts), but the bolt head and the first few mm of the bolt body were below the surface.
I tried a drill-in bolt extractor, but never seem to have luck with those. So I gave everything a good lube, (very) carefully drilled into the remaining bolt with successively wider drill bits, and then epoxied an old drill bit with a driver head into the bolt. Then very (very) gently and carefully I was able to unscrew the bolt. Aside from cosmetic damage to the top of the hole thanks to the extractor I just saved myself having to buy a new stem. RELIEF! 
As a bonus I didn’t do any damage to the screw threads either.
Lesson: double check all torque requirements!
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Can confirm that after hearing about this I had Andy also employ the same method on a rounded out bottle cage bolt on my bike. Several old drill-bits later - success!
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Very clever. The technique could probably be improved by using left handed drill bits; they’re not common but they do exist: Sydney Tools
The linked drill bits are cobalt steel: very helpful for stainless and Ti, they allow a more aggressive feed rate which gets under the work hardening issue that plagues both. Keep the tip speed below 0.25 m/s (roughly 1000 rpm for a 5mm drill) eg use the low speed on a hand held drill.
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Awesome to hear from you Mark! TBH I had never heard of left handed bits before, but makes sense that you wouldn’t want to do anything to drive the bolt in further. I guess the bolt extractor is reverse thread
Yes bolt extractors are also left hand but like you I often have poor luck with them.
I recently wrecked a thread boss on a coffee machine boiler trying to remove a broken fitting with a bolt extractor: rather than remove the fitting it just expanded it until it cracked the boss. Fortunately I was able to braze the boss back together so the machine works again. Who knew that framebuilding skills would come in so handy for coffee machines?
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