Board Thread:Questions and Answers/@comment-26242189-20160221064921/@comment-4991168-20160223163808

Basically the moment you have Alphys and Undyne befriended successfuly and unlock the True Lab you're more or less locked on the pacifist route and virtually can't screw up other than losing some fights (several are impossible to lose).

There is/was a way to abuse this by bugging the heck out of Undyne's friendship path and killing some people.