Not reserving lodging ahead of time
If you wanted to stay in an actual, like, room while you're at the Grand Canyon, your options are pretty slim. The South Rim has a few lodges and the El Tovar Hotel. And options at on the North Rim are even slimmer, with only the Grand Canyon Lodge. Book these before you even book your plane tickets -- like months in advance -- as they sell out early.
Forgetting to obtain a backcountry permit
For obvious reasons, the park limits how many people can be hiking in the backcountry at any given time. So if you're planning to camp/spend the night anywhere other than a designated campsite on one of the rims, you’ll need to get a permit. And they go insanely fast. Remember, four MILLION people visit the Grand Canyon each year. Permits become available to the public on the first of the month, five months before the month of the permit. If you don’t snag one, you may have to delay your trip another month. NOTE: this doesn't apply if you're just taking a short day hike in and out.