Joe Mauer of the Minnesota Twins lost his ability to hit sliders and curve balls decently in 2011. From 2008-2010 Joe hit .275/.327/.349 on those pitches combined. They obviously give him trouble, but if that's the worst a batter is going to hit on a pitch or pitches, he's probably doing okay. In 2011, Mauer hits .178/.186/.191 on those two pitches. What happened?
Even though Joe didn't hit all that well on those pitches, he showed decent strike zone judgment on them. From 2008-2010, he only took 28.5% of them for strikes, and chased 20.9% out of the strike zone. You can see that in his swing rate on those pitches:
In 2011, 42.5% of those pitches go for called strikes, and he's chasing 30.3% out of the zone. Again, you can see the difference in his swing rate heat map.
He's clearly taking pitches at which he should be swinging, and trying to hit too many pitches off the corners and edges. He putting a higher proportion of these pitches in play, 60.2% versus 50.2% in the three previous seasons, but because he's making contact on pitches outside the strike zone, those balls in play are turning into outs, as he has a meager .218 BABIP.
Joe Mauer's batting eye made him an MVP candidate. As far as these two pitches are concerned, he lost that eye this season, and it's responsible for a large portion of his fall off.