The head banging
One thing that happens often when adding a loft or a mezzanine is this: you bang your head at the top of the stairs. It's a thing that even architecture students do. You forget the difference in height when passing from one space to the other and it's usually at this change of heights that problems appear. Of course this is more the case of sloped roofed houses, not flat ones, though if there's is a change in roof height, you can get fooled there too. The trick is to always always draw the section of the staircase when drafting a plan. Once you're aware of this mistake, you can easily self-correct. Often, people lose sight of elements of the design that are important. It's only natural. Keeping your eyes open for forgotten details is the best way to avoid banging your head.