Edit: This bug appears in all current releases of MATHLIB (http://www.ti.com/tool/mathlib), not only C67X-MATHLIB.
I would be very surprised if there was another C67X-MATHLIB release (the last was in 2010), but I wanted to report this bug in the hope that it might help someone else down the road. The error is on line 259 of atan2dp.asm.
||[A2] stw B6, *A0 ; Y=0, _errno = EDOM if (Y,X) = (0,0)
It might take a little time to work through the assembly, but this store must not be executed unless both the Y and X arguments are 0.0 (this is indicated by the comment at the end of the line). However, this store is incorrectly executed when Y is 0.0 and X is not 0.0. In that case, A0 has not been loaded with the address of _errno (lines 246 and 248). Thus, storing B6 to *A0 corrupts whatever is in the location pointed to by A0.