One thing you can count on above everything else in the world of DIY is that a project will almost always cost more than you thought it would. Doing a detailed budget for your project is important so you can have a good idea of what you can afford. But it's tough for even experienced DIY-ers to get a really accurate estimate. Any project can throw you a curveball and you can always expect the unexpected. Especially in older homes that have been tinkered with over the years, you never can tell what a previous homeowner decided to do. Tools break and need to be replaced. You spot a small leak replacing your sink and that leads to a larger plumbing issue. You measure incorrectly and need to buy more materials. All of these issues will lead to budget overruns.
Here's an example of how a budget can get out of control: You decide to paint your dining room. You buy paint, trays, rollers and a drop cloth and think the spending is over. After discovering wallpaper underneath the paint, you decide to remove it. This leads you to the store to buy stripping tools and materials. Removing it is tough, and you mangle your walls. The only choice now is to tear the existing drywall down and replace it. Tearing down the drywall, you damage your hardwood floor, and it needs some minor repairs. You finally finish the demolition, hang new drywall, paint and fix the floor.
What started out as a $100 job has now swelled to at least five times that amount. Some budgets grow so much that you need to put the project on hold. You can avoid this by getting as detailed as possible when budgeting and adding a cushion for unexpected costs.