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In the DMG just before all the magic items there is a variant rule titled “Scroll Mishaps”
A creature who tries and fails to cast a spell from a spell scroll must make a DC 10 Intelligence saving throw. If the saving throw fails, roll on the Scroll Mishap table.
I ask, how does one fail to cast the spell in the first place?
The description of the spell scroll magic item on page 200 of the DMG says:
If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material components. Otherwise, the scroll is unintelligible.
However, if the spell is on your class spell list, but a higher level than you can normally cast, it may fail:
If the spell is on your class’s spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + the spell’s level. On a failed check, the spell disappears from the scroll with no other effect.
The description of the 3rd-level spell counterspell reads:
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.
In an unofficial tweet from December 2016, rules designer Jeremy Crawford stated:
When successful, counterspell foils a spell that is cast. The casting isn't undone. The spell fails and is wasted.
This means that the Scroll Mishap variant rule will trigger if a spell scroll is counterspelled.
