Consider these things when planning a new project...
Design Changes – Design changes during project execution almost always cause delays and cost impacts to your budget. Once the Scope document has been signed, any changes to the design need to go through your Scope Change Request Process. In my opinion, most project scope change requests that occur during the project execution phase should be declined and deferred until after the project go live date. This is due to the fact that it is difficult to properly estimate the scope and cost of new work, and most importantly the impact the new work will have on the existing work.
Skill Sets – When planning, assumptions are made (or should be) regarding people's skills. Sometimes these assumptions turn out to be wrong. Also, you will usually have people on your team who are new or have little experience. These new or lower skilled workers won't be as productive as higher skilled workers, and many times these less experienced workers extend the cost and duration of your project. Make sure your project schedule has accounted for skill levels.
Unplanned Work or Workarounds – Many times changes must be made to the sequence of planned work. These changes can impact time, cost, budget, and quality. Think about these risks up front and discuss what if any workarounds will be used. Ask why the sequence must be changed and work with the team to ensure the changes to the project occur with minimal impact to the schedule and cost.
Rework – Rework happens; it is part of most projects (especially IT). Ensure your project plan accounts for rework.
Team Morale – Turnover, project conflict, sick time, vacations all can wreak havoc with your schedule and budget; plan for these things. A happy team is a productive team. Ensure your team is working towards a common goal and not working against each other. Remove disruptive team members from your project if their behavior can't be changed.
Schedules – Trying to do too much in too little time will result in delays. Once you get behind it is very difficult (nigh, impossible) to catch up. Your project will have delays. You need to have contingency plans to get back on track quickly.
Work Environment – Ensure that your team has a proper workspace. Cramming people into poorly designed work spaces will lower productivity.
Tools – Ensure your team has the right tools to do the job. Having the right tool, but not getting into the teams hands at the right time will cause delays in your schedule.
Project Manager Overload – Too many people on a project team without the proper management oversight can cause major problems for the project manager.
Overtime – Adding hours to people's schedules in order to make a deadline will usually do nothing but increase your budget. Adding overtime rarely results in getting a late project back on track.
Executive – Executive apathy can kill your project. People are usually not going to make your project a priority if their boss isn't willing to tell them it is important.