When converting very small negative floating point numbers to strings (typically, doubles with an absolute value less than 1e-308, or similarly extreme float values), the minus sign in front of the number appears twice. For example the output might be --8.289046E-317. These are the numbers where the mantissa is unnormalized because the exponent would otherwise be below its minimum allowed value.
Applies to 9.2 and all previous releases. A patch is being produced for the 9.2 and 9.1 branches.