Opportunity Attacks

As written:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.

You can avoid provoking an opportunity attack by taking the Disengage action. You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don’t provoke an opportunity attack if an explosion hurls you out of a foe’s reach or if gravity causes you to fall past an enemy.

House rule:

In addition to the above, you take (or provoke) an opportunity attack by moving into the reach of a hostile creature and ending your turn there without taking the attack or dodge action.

Critical Fail on Attack Roll

Any natural 1 on an attack roll will trigger “friendly fire” — the attack will harm oneself or a friendly within range (melee) or line of sight (ranged attacks). Roll damage as normal. This rule applies to everyone, including monsters and adversaries.