When my mum came down with Alzheimer's, she forgot how the abstract concept of left worked.
I'd heard of the problem (inability to perceive a side) existing in rare cases before she got ill, but it's such a bizarre thing that I had assumed it had to be misreporting before I finally saw it: she would eat food on the right side of her plate leaving the food on the left untouched, insist the plate was empty, but rotating the plate 180 degrees let her perceive the food again; she liked to draw and paint, so I asked her to draw me, and she gave me only one eye (on her right); I did the standard clock-drawing test, and all the numbers were on the right, with the left side being empty (almost: she got the 7 there, but the 8 was above the 6 and the 9 was between the 4 and 5).
When she got worse and started completely failing the clock drawing test, she also demonstrated in multiple ways that she wasn't able to count past five.