Line segments are joined wrongly by Ghostscript 8.71 under the following circumstances:
1) round join is selected AND
2) interpolation is requested AND
3) stroking path contains consequent parallel segments.
This way, the join of the parallel segments is rendered funny.

PostScript code to reproduce the error:
1 setlinejoin 6 setlinewidth
0 0 moveto 50 50 rlineto 50 50 rlineto stroke

View that with -dGraphicsAlphaBits=4 or 2.

The first segment will end with a round cap, and the second one will become triangular with width increasing linearly from 0 to 6 along it. PostScript Language Reference (3rd edition) says nothing special about parallel segments in a path, so this is clearly a misbehaviour of Ghostscript's stroking routine (perhaps, something like division by zero).

Tested on GPL Ghostscript 8.71 (2010-02-10) for Win32 on Windows 2000 SP4.