Or in other words: What rules prevent you from doing something you would otherwise be able to do?
I am worried about this question being too broad, but I feel like there can't be very many examples like the following:
Under the rules on bonus action spells, it states:
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
First, this means that if you cast a bonus action spell, the restrictions on them prevent you from casting non-cantrip spells. However if instead you cast a non-cantrip spell the restrictions on bonus action spells prevent you from casting any bonus action spells. You are normally allowed to cast a bonus action spell, but here, their own rules prevent that. That this is how bonus action spells work is shown here
Under the net weapon, the "special" property states:
When you use an action, bonus action, or reaction to attack with a net, you can make only one attack regardless of the number of attacks you can normally make.
First, if you have Extra Attack, and your first attack is with a net, the restrictions on nets prevent you from making more attacks, as shown in this question. However, this question, along with the previous one, possibly show that if you instead make your first attack with, say, a dagger, the restrictions on nets prevent you from attacking with a net. You are normally allowed to attack with a net, but here, its own rules prevent that.
Are there other examples of times similar to these?