If a hexadecimal constant follows directly after an opening parenthesis without being separated from it by whitespace, Qalculate! fails to parse the expression.
For instance, using the "qalc" command-line tool, the expression
error: "ff" is not a valid variable/function/unit.
0 * x = 0
yields an error, while
> ( 0xff)
255 = 255
(note the space) works without problems.