Parkinsons's law

"Work expands so as to fill the time available for its completion."

This ties into other time management concerns, like procrastination.

Often a simple bug fix can grow in scope, or the work is delayed until the end of the sprint.

Some strategies to combat Parkinson's law:

Online your vision statement and drivers. Why are we doing this work?
Clarify roles and responsibilities. Avoid 'social loafing'.
Understand what's in scope - avoid scope creep
Set a timeline in advance. Have milestones and deadlines and try to stick to them.