God, pretty much everything... lots of failures involving mince. And overcooked vegetables... and things like lamb's brains. Yuck. Yuck. Yuck.
Two frequent highlights were:
"Stir fry" with strips of no doubt quite expensive beef cooked until it was like rubber, with overcooked non-asian vegetables, watery vegetable water, and pasta shells...
Grilled sausages that had been pre-boiled, having been pricked all over to let the fat out. this inevitably meant the sausage mince was grey, tasteless, gagworthy little bits, and the skins would fall off under the griller, and the whole thing would be served up with grilled onions. Onions make me gag, always have, always will...
I was nearly 30 before i dared try a sausage and discovered that they're quite nice with the fat still in! 
But i'm being mean complaining. Mother has ASD*, has practically no sense of taste or smell whatsoever, is completely insensitive to textures in food, and follows rules obsessively. So for her, the fact she followed the recipes for every single thing in the meal, means she got it right. Usually the recipe-following is fairly inaccurate though, so the recipes don't usually work... 
*as do I, but my sensory issues are kind of mirrors of hers...