I'm reporting a possible bug re the discover/movie/with_genre variable. The genreId value of 27 is being passed to the function, and this is informed as "horror". However, the following request:

http://api.themoviedb.org/3/discover/movie?api_key=API&with_genre=27&sort_by=popularity.desc&language=en&include_adult=false&include_video=false&release_date.lte=2015-01-19&release_date.gte=1950-01-01&page=21

returns the following response which includes non-horror genre items, including Meet the Parents which is listed as comedy and romance as per the web site:

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Fockers","release_date":"2010-12-21","poster_path":"/vpVa9AaUHokKoRzDQmEPGgAWpaX.jpg","popularity":2.12961821375012,"title":"Little Fockers","video":false,"vote_average":5.5,"vote_count":380},{"adult":false,"backdrop_path":"/i5r9aTDKo1y6paUX1PHsPhZstZk.jpg","id":113833,"original_title":"The Normal Heart","release_date":"2014-05-25","poster_path":"/fIf4nLpWHK8BsbH76fPgMbLSjuU.jpg","popularity":2.12830736195653,"title":"The Normal Heart","video":false,"vote_average":7.9,"vote_count":66},{"adult":false,"backdrop_path":"/kCF76wyci4fe4DYsOIRLq1KI92t.jpg","id":178809,"original_title":"The Colony","release_date":"2013-09-20","poster_path":"/4E1jXxHcUzAQYOTlFCIANMg29YA.jpg","popularity":2.12759859382851,"title":"The Colony","video":false,"vote_average":5.3,"vote_count":167},{"adult":false,"backdrop_path":"/asTq74kXjslx8A13qDTVdzvyJ8.jpg","id":127517,"original_title":"Disconnect","release_date":"2012-09-11","poster_path":"/kTLcEBTPzptlBP8auLRlK8KTKvV.jpg","popularity":2.12630704681587,"title":"Disconnect","video":false,"vote_average":6.9,"vote_count":123},{"adult":false,"backdrop_path":"/my81Hjt7NpZhaMX9bHi4wVhFy0v.jpg","id":187,"original_title":"Sin City","release_date":"2005-03-31","poster_path":"/eCJkepVJslq1nEtqURLaC1zLPAL.jpg","popularity":2.41330865030552,"title":"Sin City","video":false,"vote_average":7.0,"vote_count":828},{"adult":false,"backdrop_path":"/jU5iViCqvycEnmbX04kYZi10lnR.jpg","id":52451,"original_title":"The Three Musketeers","release_date":"2011-10-20","poster_path":"/n8ob0xjLIjnvqgWY57rPQYOw7t.jpg","popularity":2.12187091067186,"title":"The Three Musketeers","video":false,"vote_average":5.7,"vote_count":292},{"adult":false,"backdrop_path":"/C9Fiv2lZOuQEWWuXB8C0LVZapC.jpg","id":138843,"original_title":"The Conjuring","release_date":"2013-07-19","poster_path":"/xYGkdhOEVoFrLNUotuTXemtQAWd.jpg","popularity":2.11895559984499,"title":"The Conjuring","video":false,"vote_average":7.3,"vote_count":864},{"adult":false,"backdrop_path":"/eyiE1vuizMKVu8MoZkg2xsramFl.jpg","id":49444,"original_title":"Kung Fu Panda 2","release_date":"2011-05-25","poster_path":"/9wCrh57WO3mPmn1hWnJRcvdFtIb.jpg","popularity":2.11881664258398,"title":"Kung Fu Panda 2","video":false,"vote_average":6.5,"vote_count":620},{"adult":false,"backdrop_path":"/vcLqVuSMx7dVfuOVKd4IL4RcMsa.jpg","id":114150,"original_title":"Pitch Perfect","release_date":"2012-09-28","poster_path":"/qtKYrSlnrgLS7wrYvEbcxrtAAqa.jpg","popularity":2.11683085830515,"title":"Pitch Perfect","video":false,"vote_average":6.9,"vote_count":567},{"adult":false,"backdrop_path":"/ekLAySalu0CGoGogPWY0BG6uAul.jpg","id":64690,"original_title":"Drive","release_date":"2011-09-16","poster_path":"/fsnaaNQ4RmZwgRv6qfT7NIi41Sj.jpg","popularity":2.1136803784742,"title":"Drive","video":false,"vote_average":6.9,"vote_count":988},{"adult":false,"backdrop_path":"/nJaVxQNC4pb6eYH5jv0UUokfH5X.jpg","id":8467,"original_title":"Dumb and Dumber","release_date":"1994-12-16","poster_path":"/st4P2TtPrAfNwu8HLXoPsPPii42.jpg","popularity":2.20195072797815,"title":"Dumb and Dumber","video":false,"vote_average":6.6,"vote_count":307},{"adult":false,"backdrop_path":"/62xHmGnxMi0wV40BS3iKnDru0nO.jpg","id":109091,"original_title":"The Counselor","release_date":"2013-10-25","poster_path":"/uxp6rHVBzUqZCyTaUI8xzUP5sOf.jpg","popularity":2.10877477834009,"title":"The Counselor","video":false,"vote_average":5.2,"vote_count":219},{"adult":false,"backdrop_path":"/ma5Ps40txofscTtWpjs39swMnwZ.jpg","id":18823,"original_title":"Clash of the Titans","release_date":"2010-04-02","poster_path":"/n8W2Y72VzSi8Yz6IvYWwfoiMTS6.jpg","popularity":2.10271895622764,"title":"Clash of the Titans","video":false,"vote_average":5.7,"vote_count":921},{"adult":false,"backdrop_path":"/bGXup8uvaIrp6mLisZ8AJtl6DNA.jpg","id":561,"original_title":"Constantine","release_date":"2005-02-18","poster_path":"/q19U91KJ1yabMmFYSyu5U7SetsP.jpg","popularity":2.6691570326514,"title":"Constantine","video":false,"vote_average":6.4,"vote_count":509},{"adult":false,"backdrop_path":"/gulyiqtyuzFRJCnvNwD6Z786kQi.jpg","id":44912,"original_title":"Green Lantern","release_date":"2011-06-16","poster_path":"/zUSEYp9G7Hk9GZIfbPoOiSHvwHV.jpg","popularity":2.08867936943242,"title":"Green Lantern","video":false,"vote_average":5.2,"vote_count":905},{"adult":false,"backdrop_path":"/iMBlLEjC74OQbQ60bubdF27P35K.jpg","id":1597,"original_title":"Meet the Parents","release_date":"2000-10-05","poster_path":"/omw5eaaPFgIo3IGJBbAF5xIiMLp.jpg","popularity":2.29405481084095,"title":"Meet the Parents","video":false,"vote_average":6.5,"vote_count":385},{"adult":false,"backdrop_path":"/bv3AR0Y0d4So1OODqpwB22J9Rkw.jpg","id":305668,"original_title":"Sunny Side Up","release_date":"2015-01-17","poster_path":"/klSM5Y11ZwK5W2loq8958dM8ZKt.jpg","popularity":2.08375902930998,"title":"Sunny Side Up","video":false,"vote_average":0.0,"vote_count":0}],"total_pages":8300,"total_results":165988}

Here is the collection of genre IDs I am returned:

{"genres":[{"id":28,"name":"Action"},{"id":12,"name":"Adventure"},{"id":16,"name":"Animation"},{"id":35,"name":"Comedy"},{"id":80,"name":"Crime"},{"id":99,"name":"Documentary"},{"id":18,"name":"Drama"},{"id":10751,"name":"Family"},{"id":14,"name":"Fantasy"},{"id":10769,"name":"Foreign"},{"id":36,"name":"History"},{"id":27,"name":"Horror"},{"id":10402,"name":"Music"},{"id":9648,"name":"Mystery"},{"id":10749,"name":"Romance"},{"id":878,"name":"Science Fiction"},{"id":10770,"name":"TV Movie"},{"id":53,"name":"Thriller"},{"id":10752,"name":"War"},{"id":37,"name":"Western"}]}