Decode A Dishwasher Error Code Before You Call An Engineer

The display's flashing a code you've never seen and there's a puddle forming under the door. This explains what the code usually means, whether it's safe to check and clear yourself, and exactly when to stop fiddling and call an appliance engineer instead.

Explain what error code [code, e.g. E15] on a [dishwasher make and model] means. Say what usually causes it, whether it's something safe to check and try to clear myself, and at what point this becomes a job for a qualified appliance engineer rather than something to keep resetting.