Here are some errors I've found while playing through Pokémon Y:
Error: Pictures of Flabébé's alternate forms are not all in the same pose.
Link: www.serebii.net/pokedex-xy/669.shtml
Error: Nincada locations missing "In-Depth Details" and "Details" links.
Link: www.serebii.net/pokedex-xy/290.shtml#location
Error: Gorebyss locations missing "In-Depth Details" and "Details" links.
Link: www.serebii.net/pokedex-xy/368.shtml#location
Error: Route 7 horde Psyduck Max Level says 7, but I've only found level 8 Psyduck hordes.
Link: www.serebii.net/pokearth/kalos/route7.shtml#horde
Error: Route 10 Yellow Flowers Snubbull Min Level says 20, but I've found a level 19 Snubbull.
Link: www.serebii.net/pokearth/kalos/route10.shtml#flowers
Error: Route 9 Helioptile Min Level says 16, but I've found a level 15 Helioptile.
Link: www.serebii.net/pokearth/kalos/route9.shtml#grass
Error: Route 14 Good Rod Poliwhirl Max Level says 25, but I've found only found level 35 Poliwhirl.
Link: www.serebii.net/pokearth/kalos/route14.shtml#goodrod
Error: Route 17 Sneasel Min Level says 39, but I've found a level 38 Sneasel.
Link: www.serebii.net/pokearth/kalos/route17.shtml#grass
Error: Route 19 Super Rod Poliwhirl and Whiscash Min Level both say 50, but I've found only found level 45 Poliwhirl and Whiscash.
Link: www.serebii.net/pokearth/kalos/route19.shtml#superrod
Error: Exeggcute locations missing Azure Bay as shown in details:
www.serebii.net/pokedex-xy/location/102.shtml.
Link: www.serebii.net/pokedex-xy/102.shtml#location
Error: Chinchou locations missing Shalour City as shown in details:
www.serebii.net/pokedex-xy/location/170.shtml.
Link: www.serebii.net/pokedex-xy/170.shtml#location
Error: Remoraid locations missing Shalour City as shown in details:
www.serebii.net/pokedex-xy/location/223.shtml.
Link: www.serebii.net/pokedex-xy/223.shtml#location
Error: The area to click on the Kalos map for Frost Cavern is not in the right place.
Link: www.serebii.net/pokearth/
Error: Terminus Cave Rock Smash encounter percentages do not add up to 100%.
Link: www.serebii.net/pokearth/kalos/terminuscave.shtml#rocksmash
Error: Ursaring page lists Victory Road as a location, but the neither the details page nor the Victory Road page show Ursaring.
Link: www.serebii.net/pokedex-xy/217.shtml#location
Error: Spearow locations has a "Details" link for Y but not for X even though their details are the same.
Link: www.serebii.net/pokedex-xy/021.shtml#location
Error: Spearow location details claims can be found outside Frost Cavern but the Spearow page says breed and the Frost Cavern page has no mention of Spearow.
Link: www.serebii.net/pokedex-xy/location/021.shtml
Error: There is a "[A]" after Pumpkaboo and Gourgeist in the Mountain Kalos dropdown box.
Link: www.serebii.net/pokedex-xy/
The following I can't prove are wrong, but I believe they are errors:
Error: Route 22 claims level 6 Farfetch'd can be found, but I've only found level 7 Farretch'd.
Link: www.serebii.net/pokearth/kalos/route22.shtml#grass
Error: Shalour City Surfing claims level 25 Mantyke can be found, but I've only found level 27 Mantyke.
Link: www.serebii.net/pokearth/kalos/shalourcity.shtml#surf
Error: Glittering Cave - 1F claims level 15-17 Onix can be found, but I've only found level 17 Onix.
Link: www.serebii.net/pokedex-xy/location/095.shtml
Error: Glittering Cave - 3D Area claims level 15-17 Lunatone can be found, but I've only found level 17 Lunatone.
Link: www.serebii.net/pokedex-xy/location/337.shtml
Error: Glittering Cave - 3D Area claims level 15-17 Solrock can be found, but I've only found level 17 Solrock.
Link: www.serebii.net/pokedex-xy/location/338.shtml
Error: Glittering Cave - 3D Area claims level 16-17 Rhyhorn can be found, but I've only found level 17 Rhyhorn.
Link: www.serebii.net/pokedex-xy/location/111.shtml