Many people use their most optimistic estimate of time to decide how long something should take, instead of the conservative or midrange estimate. Most also don't include extra time for things like slow days, errors, late tasks, and the like.
Other examples can include:
- A task may take more than once to get right;
- People under estimate issues like change management and putting corrective actions in place;
- People can concentrate on the critical path tasks, and then forget to look at the problem holistically, then something of the critical path then comes onto the critical path;
- teams forget the single point of failures - people going off ill, on holidays, moving roles, leaving the company, training days and don't have any fallback options in place;
- When people are doing multiple tasks/matrix roles, it can take them some time to get back in the grove to be fully productive;
- People forget to plan things or take things into account, which then requires more time to put it right;
- You can't plan for everything, and sometimes unexpecting things either within your control or outside your control do happen, and you must adjust;
- Taking new people onto the project is always difficult in terms of training them up and the team building that is necessary to bring them into the team;
- People are people, and they will always do unexpected things.
