A foul up. If someone is not doing their job correctly. A mistake. An unfortunate mishap. An event that disrupts the activity of others.
A man driving a truck runs over a speed bump and passengers are unaware of the speed bump so that was a derp. Someone is neglegent on the job and a colleague yells "derp"!. Someone gets an infection in a third-world country. I got an infection in another country so that was a derp. Someone does not tend to required tasks of their job which leads to foul ups. I should have been paying attention so that was a derp.