What are some examples of incorrect data formats that can cause VBA Error 400 when using XMLHTTP POST requests?

Common mistakes include: 1) Incorrect Content-Type header: Ensure it matches the data format (e.g., `application/json` for JSON data). 2) Missing or malformed JSON or XML structure: Use a validator to ensure your data is valid. 3) Encoding issues: Use UTF-8 encoding for data containing special characters. 4) Incorrect date or number formats: Ensure they match the server's expected format. 5) Sending empty or null values when they're not allowed.