It can start small: project A is predicted to not be full time, so you are also assigned to project B. But at the first obstacle (or the first wishful-thinking

It can start small: project A is predicted to not be full time, so you are also assigned to project B. But at the first obstacle (or the first wishful-thinking based estimate or plan) you don't get enough done on either project, overtime is needed, and it's your fault. Then project C takes the sales team by surprise, and who's going to deliver it?