Choosing too many fonts, including low-resolution images, using text that's too small or difficult to read, and not adding enough white space. It’s best to stick to two or three fonts at the most and make sure they complement each other. Low-resolution images often look blurry and pixelated when enlarged, so it's wise to use high-resolution images whenever possible. White space is critical for creating a visually appealing layout and making text easy to read.