Keep in mind if you cast a spell using a bonus action, the only other spell you can cast on your turn is a cantrip. This means no counterspell if you have already cast a spell using a bonus action on that turn. It doesn't happen very often, but it is possible.
Just to be clear...It can really only happen on your own turn (since that is when you get a bonus action); you would have already cast, or be casting, a bonus action spell; the enemy is now either countering the bonus action spell, or a cantrip you cast after the bonus action spell; you cannot counterspell the counterspell. It may also occur during a spell based legendary action, since they occur at the end of other creature's turns.
I don’t think that’s actually the rule but I’m welcome to being proven wrong
From PHB: Casting Time: Bonus Action: 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.
If someone there is another rule that overrides the rule above, I am unaware of it.
Can you also cast a reaction spell on your turn?
You sure can! Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.