OS - Windows 10 64 bit; CPU - Third-gen i7; GPU - ATI HD 7xxx; RAM - 24 gigs
Broken: Hash a58a8eb (May 3)
Worked: Hash 32e2036 (April 27)
Short description of error
The spin tool in recent builds can only extrude properly on the XY plane now. It is unable to create faces traversing the XZ or YZ planes and in those cases will either create highly skewed faces or zero-area faces.
Exact steps for others to reproduce the error
Open the .blend and use the spin tool from that point of view (leaving the cursor where it is), notice all of the new zero-area faces perfectly aligned with the starting one on the XY plane.