A caster can no longer 'Counterspell', with a spell slot, the 'Counterspell' that targets their leveled spell cast with a spell slot, since they would then be using two spell slots per turn.
A character who casts a spell with a slot and then falls into a pit trap on their movement can no longer cast 'Feather Fall' to stop the falling damage, as that would be two spell slots per turn. An almost identical character with the Magic Initiate Feat can cast 'Feather Fall' if that is what they picked as their daily slotless Level one spell.
A character who casts a spell with a slot and then falls into a pit trap on their movement can no longer cast 'Feather Fall' to stop the falling damage, as that would be two spell slots per turn. An almost identical character with the Magic Initiate Feat can cast 'Feather Fall' if that is what they picked as their daily slotless Level one spell.