«Vieux» might be considered an exception to this rule, as the adjective has 2 masculine singular forms, but the feminine singular form (vieille) always follows the form of «vieil» , regardless of whether followed by a vowel/mute h or a consonant. I don't know of any others - does anyone else?
Have a look at the video in the lesson, there is an explanation on the adjectives (including ‘vieux’) which don’t follow the rule. However, they are all different to adjectives ending in -eux like ‘vieux’.
There are two more adjectives (but they don’t end in -eux either):
mad: fou(x) / fol / folle(s)
ginger: roux / rousse(s)
I hope this is helpful.
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