This code issue is minor because it is not currently triggered by existing code.

StringBuilder uses a BufBuilder (bson/util/builder.h) to manually manage a char buffer. The default copy constructor and operator= will shallow copy the data pointer, which will ultimately lead to freeing the memory multiple times. You can produce the issue with:
{
StringBuffer a;

{ StringBuffer b=a; }

// b d'tor frees b.data, which == a.data

{ StringBuffer c(a); }

// c d'tor frees a.data again

} // a d'tor frees a.data again