Just as addition of the float type is inaccurate, multiplication of the decimal types can be inaccurate (or cause inaccuracy) if you exceed the precision. See Data Type Conversion and decimal and numeric.
Since you multiplied NUMERIC(24,8) and NUMERIC(24,8), and SQL Server will only check the type not the content, it probably will try to save the potential 16 non-decimal digits (24 - 8) when it can't save all 48 digits of precision (max is 38). Combine two of them, you get 32 non-decimal digits, which leaves you with only 6 decimal digits (38 - 32).
Thus the original query
SELECT A, B, C, A + B * C
FROM ( SELECT CAST(0.12345678 AS NUMERIC(24,8)) AS A,
CAST(0 AS NUMERIC(24,8)) AS B,
CAST(500 AS NUMERIC(24,8)) AS C ) T
reduces to
SELECT A, B, C, A + D
FROM ( SELECT CAST(0.12345678 AS NUMERIC(24,8)) AS A,
CAST(0 AS NUMERIC(24,8)) AS B,
CAST(500 AS NUMERIC(24,8)) AS C,
CAST(0 AS NUMERIC(38,6)) AS D ) T
Again, between NUMERIC(24,8) and NUMERIC(38,6), SQL Server will try to save the potential 32 digits of non-decimals, so A + D reduces to
SELECT CAST(0.12345678 AS NUMERIC(38,6))
which gives you 0.123457 after rounding.
