Add input rules and character count limit to the text box: 

 

1. There is no input mask, although the expected pattern is AARRGGBB.
2. Arbitrary and invalid characters can be input (#, +, %, Z, ...).
3. Invalid input is parsed to FF000000, but there is no indication that the input is wrong.
4. Shorter valid sequences, e.g. AB, are parsed, but to FFAB0000, instead of AB000000, which seems unintuitive.
5. Longer sequences exceeding the pattern can be input, although invalid.
6. Entering longer sequences leads to the expansion of the input box even beyond the bounds of the control