Pseudoautosomal regions of the X chromosome showing heterozgyosity
A normal male can show heterozygosity in the "X" results of a 23andMe SNP test despite having only one X chromosome.  This is because there are very short regions at the ends of the Y chromosome, called pseudoautosomal regions, that match up with equivalent (homologous) regions at the ends of the X chromosome.  

Because the pseudoautosomal regions of the Y are homologous to the matching regions of the X, the SNP results for those areas cannot be distinguished in the SNP tests, so the results appear along with the X results in "X" chromosome file.  The results in those two areas of the file can be either homozygous or heterozygous at each location.  Most of the locations in the "X" file are for  locations outside of those regions, so most of the file entries cannot report heterozygous results.