is a low-level box construct that represents boxes that cannot be interpreted in input or output.
- ErrorBox[boxes] typically displays as the raw form of boxes together with underlining that indicates parts that cannot be interpreted.
- An explicit ErrorBox is typically generated by a failed call to MakeExpression.
- When the front end reads an invalid box structure from a Notebook or Cell expression, it generates an implicit ErrorBox, but it preserves the original structure when the expression is written out.
- ErrorBox is displayed as a pink box. The pink box may have a tooltip or contents that help to indicate the nature of the error.