Stress
There's some pressure to prove yourself useful. In other words, if you're spending all day testing the best way to package a perishable product in order to maximize freshness, and you come back with the idea of storing them in Fabergé eggs, you may get laughed all the way to the unemployment office.
If you're a certain type of engineer, such as the petrochemical engineer in our example, you'll stress about being physically injured while on the job (you'd have to supervise drilling as well as handle all the office and science-y stuff). But, on the whole, the stress in this job is pretty low...unless, of course your experiments aren't working, in which case you'll likely be tearing your hair out.