The documented values for SqlDecimal.MaxValue and MinValue are incorrect.
The documentation states that the values are 79,228,162,514,162,514,264,337,593,543,950,335 and -79,228,162,514,264,337,593,543,950,335 respectively.
The actual values of these constants (according to the ToString() method and Data property) are 99,999,999,999,999,999,999,999,999,999,999,999,999 and -99,999,999,999,999,999,999,999,999,999,999,999,999.
The documentation states that the values are 79,228,162,514,162,514,264,337,593,543,950,335 and -79,228,162,514,264,337,593,543,950,335 respectively.
The actual values of these constants (according to the ToString() method and Data property) are 99,999,999,999,999,999,999,999,999,999,999,999,999 and -99,999,999,999,999,999,999,999,999,999,999,999,999.
I've opened a bug for this issue at MSDN.