What is wrong with مَرَرْتُ بِالْمَسَاجِدَ?

Ghayr Munsarif: Basic Case Pattern and Exceptions

• The noun should take kasrah because al is present: بِالْمَسَاجِدِ.
• The preposition should be removed because a diptote cannot be genitive.
• The noun must take tanwin fath because al is present.
• Nothing is wrong; a diptote always has fathah in the genitive.