programming error: stopped tuplet bracket has left nor right bound

[Nick Payne]

I'm getting this error in the console output - no indication is given of 
where in the source is causing the error. The PDF output is fine. I did 
a google search for the error message but didn't find anything that 
seemed relevant. There are tuplets all the way through this score, so 
after showing the tuplet numbers and brackets for the the first bar, I set

\override TupletBracket #'stencil = ##f
\override TupletNumber #'stencil = ##f

to turn them off. I'm now up to bar 42, and the error first showed a 
couple of bars back. What does it mean? using v2.13.27.

Nick