Doesn't know how to debug. Doesn't know why trivial bugs happen.
Examples for front-end dev (CSS/HTML/JS)
Uncleared float causing a collapse, collapsing margins, poor selector priority or confused about why X has priority over Y. Div soup structure with unnecessary divs/floats to attain desired layout. Off-by-one errors in Javascript (without being able to identify it) or not being able to identify when they are out-of-scope or what `this` is referencing.
Once they stop making silly mistakes they become a `developer'` At which point the kinds of mistakes they are making are less trivial and occasionally overlap with problems `senior developer` have. But they can confidently debug and generally solve their own problems so require less attention from other devs.