1) Used `class` attribute in JSX instead of `className` (Header element).
2) Mismatched/unclosed root `div` tags causing unbalanced JSX/HTML.
3) HTML comment (`<!-- -->`) placed inside JSX (invalid in JSX).
4) `<img>` tag not self-closed in JSX (should be `<img ... />`).
5) Used `onclick` (lowercase) instead of `onClick` on a JSX element.
6) Used wrong variable names in map: `itm` vs `item` causing ReferenceError when spreading.
7) Misspelled `setState` as `setstate` (method does not exist).
8) Used `ReactDom` (capitalization error) instead of `ReactDOM`.
9) Script type set to `text/javascript` for JSX code — should be `text/babel` when using Babel standalone.
10) Incorrect usage of spread / undefined variable when passing props: `{...item}` where `item` is not defined in that scope.