Not cleaning your makeup brushes.
We've all been guilty of this from time to time. Even most makeup artists confess that the time consuming brush cleaning process is their least favourite part of the job. But even if you don't care that dirty brushes can make you break out, don't forget that they're affecting your makeup application. Not only will the colours be all muddled together, but a precise technique is rendered way more complicated — the bristles are already overloaded with pigment, blurring that perfect cut crease to oblivion.