You move sideways to get a clear view of an enemy, just as a teammate begins firing. Neither player intended to hit the other, but their plans occupied the same strip of space.
Friendly fire changes how a co-op team uses a room, corridor or open field. A position that gives one player a clear attack may place another directly in danger.
The exact rules depend on the game. Arrowhead describes friendly fire as a core part of Helldivers. Other games may offer different settings or exceptions, so check the mode you are actually playing rather than carrying assumptions between titles.
Agree on a few short calls
For a regular group, try three plain messages during an easy encounter: announce that you are crossing in front, identify the area of a planned attack, and say when you are moving back. Use wording your teammates can understand quickly. Long explanations compete with the information already coming from the game.
A warning should arrive before the action. Calling out an explosive effect after it has landed gives the team little opportunity to respond. Equally, a teammate needs time to acknowledge or move; an announcement does not make an unsafe action automatically sensible.
Leave an exit, not just a firing position
Look at a difficult encounter from above in your mind. Where can each player retreat? Is someone trapped between a wall and another player’s attack area? Can the group spread out without losing the ability to help one another?
During a quieter practice, take turns moving while the other players hold their positions. Watch how often a teammate crosses an occupied line or stands in a planned effect. This is an observation exercise for a videogame team, not a requirement to copy any particular formation.
After an accidental hit, identify the sequence rather than assigning blame: the warning came late, the crossing was unexpected, or the group had no clear escape space. Change one habit for the next attempt. Deliberately damaging teammates undermines that learning process; if it continues, use the game’s moderation and session controls instead of treating it as normal co-op play.