One of the assumptions of linear regression is that the residual mean is zero. As far as I can tell though, the residual mean is always zero i.e. it is not an assumption, it is a fact.
The formula for calculating the least squares line means that the sum of all residuals is zero and the mean of all residuals is zero (even if the line is not actually linear).
Can someoen explain if/why this is not the case? Would appreciate an "english" response rather that jumping straight into the maths please!