Dark slide error would manifest on the side, not top I would have thought. Check play there though. Same with end of reel too. Unless you were shooting on its side (unlikely!).
I wonder if it is light seals in the film back. The last frame is most vulnerable as the back is removed for final winding and film removal - the rest of the frames are protected within the back and by the camera body.
Leica M2, Nikon D700, Bronica RF645, Leica CL, Summicron 40mm, Rolleicord Va, Hasselblad 500 CM Zeiss Planar, Leica 50mm Summicron V3, Hasselblad PME51 metered prism, Zeiss Ikon Super Ikonta 534/16 & Ensign 820 Special - all gone.