As soon as a project involves other team members you need to take into account their time constraints. Some of my projects involve scheduling time with faculty members to assist them with a particular issue they are having. These projects are obviously then subject to their schedules and other commitments.
I am very careful to get my projects done on time (to scope) but occasionally scope creep sets in and my project can end up taking longer than I thought it would. One of the projects I am working on at the moment involves assessing the state of the interactive whiteboards at the school and making a recommendation to purchase replacement boards within the next budget period. This process is taking longer than expected as it is difficult to get access to the boards to test them (they are being used all day). In itself, the fact that they are being used all day should mean that they are working fine, however that isn't the case. Many of the teachers are just 'making do' with the functionality that they have. There isn't enough communication from the teachers about the issues they are experiencing with the boards. Therefore my team and I have to throughly test each board after hours which isn't happening quickly enough. It seems in this case there are two things causing scope creep: lack of communication and accessibility.