The GATK can be particular about the ordering of a BAM file. If you find yourself in the not uncommon situation of having created or received BAM files sorted in a bad order, you can use the tool ReorderSam to generate a new BAM file where the reads have been reordered to match a well-ordered reference file.
java -jar picard/ReorderSam.jar I= lexicographc.bam O= kayrotypic.bam REFERENCE= Homo_sapiens_assembly18.kayrotypic.fasta
This tool requires you have a correctly sorted version of the reference sequence you used to align your reads. This tool will drop reads that don't have equivalent contigs in the new reference (potentially bad, but maybe not). If contigs have the same name in the bam and the new reference, this tool assumes that the alignment of the read in the new BAM is the same. This is not a lift over tool!
The tool, though once in the GATK, is now part of the Picard package.