End game deciding on dead stones
As TimK pointed out, the situation could be but may not be a Seki but without a diagram to show to us, it's not easy for us to guess what happened.

Seki : no one die, everyone live

$$cm1
$$ +---------------------------------------+
$$ | X . O X . . . . . . . . . . . . . . . |
$$ | X . O X . . . . . . . . . . . . . . . |
$$ | X X O X . . . X . . . . . . . . . . . |
$$ | O O X X . . . . . . . . . . . . . . . |
$$ | . . O O X X . . . . . . . . . . . . . |
$$ | . O . . O . . . . . . . . . . . . . . |
$$ | . . . O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |


Some patterns may look like a Seki but can be a simple Bulky Five for example. In this case, even if both opponents can attari it is not a seki.

Bulky-five : Black dies after capturing because he cannot make two eyes.

See this variation on eidgogo to explain why the bulky five is dead.

$$cm1   
$$ +---------------------------------------+
$$ | . . O X . O . X O . . . . . . . . . . |
$$ | . . O X . . X X O . . . . . . . . . . |
$$ | . . O X X X X O O . . . . . . . . . . |
$$ | . . O O O O O O . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |


There's also a Bent-three : Black dies

$$cm1   
$$ +---------------------------------------+
$$ | . . O X X X X O . . . . . . . . . . . |
$$ | . . O X O . X O . . . . . . . . . . . |
$$ | . . O X . X X O . . . . . . . . . . . |
$$ | . . O X X X O O . . . . . . . . . . . |
$$ | . . O O O O O . . . . . . . . . . . . |