I've recently started DM-ing 5e for the first time.
Rogue's sneak attack says that the sneak attack bonus damage is applied once per turn to a selected attack the Rogue has an advantage on.
DMG, p.251 - as admittedly optional rule - states that flanking gives advantage. I'm willing to allow it, because it just makes a lot of sense, especially if the enemy is exactly between two players.
The situation: In a narrow corridor a Rogue and druid stand next to each other, together occupying corridors width. A monster (medium size) approaches them and starts attacking. Rogue uses his move action to move behind him (never leaving enemy's reach, so no AoO), and therefore they're flanking - sandwiching - the monster. By flanking rules, they now have advantage on the attack rolls, and Rogue can apply his sneak attack damage.
My question: is that scenario correct, or am I missing some important rule that makes this invalid? I don't mind my players having a lot of damage output, I think that it makes sense that a surrounded enemy is getting stabbed to death, but I don't want all my calculated encounters be completely out of balance now.