I went to a different gym this week for a spin class and the instructor could not clip into her bike (newbie cleat user, she said). All of the participants wearing cleats had Shimano SPDs (or same-design knockoffs). She bought SPD-compatible cleats, too, according to her receipt, but simply could not clip into the pedals. I looked: they were not upside down or backwards... but the shape was new to me (and to other old-timers wearing cleats).
Mine, labeled SH-71 (and another pair labeled SH-98) have a "beak" at the front. It's pointed and has a bevel that allows easy insertion into the pedal. Hers were completely flat (no bevel) and the nose was rounded instead of pointed -- apparently Shimano's SM-SH56 and/or SM-SH51 design.
I had no problem clipping into one of the pedals, so it wasn't a matter of high tension. I couldn't clip her shoe in even when on the floor looking at the alignment from the side.
So my question is, did Shimano redesign the pedal/cleat system within the 7 or 8 years since I last bought SPD cleats? Or is it possible that she just got a bum pair of cleats (from Amazon)? Could the pedals be an old design and not forward-compatible with the rounded nose on the cleats? I sent her to an LBS to have someone look at them, but I'm still curious.