Not Giving Yourself Enough Time
Besides added costs, another thing you can count on if you're tackling a job yourself is that it will almost always take longer than you think it will. Most people engaging in a home renovation aren't able to do so full-time. They have jobs and become DIY weekend warriors, or they take "staycations" and attempt to get a project completed in a week or two. Professional contractors tend to take longer, so it's highly unlikely that a DIY-er will finish sooner than expected or even on time. In fact, in the DIY world, "on time" becomes a sliding scale.
This can be a problem because if the project isn't finished in the time you've carved out for it, life can get in the way and you can end up with a half-finished bathroom for days, weeks or even months. If you're dealing with plumbing or electrical issues, you could end up without running water or power in your house for a period of time as well. Avoid short-changing yourself on time by having realistic renovation goals. If you're not sure how long a task should take, ask someone who knows or do some surfing on DIY message boards on the Internet. A knowledgeable employee at your local hardware store can also give you an idea of how much time you'll need. And finally, get a friend to help you and then help him or her in return. Working with a partner will always speed things up.