Non properly sorted trees

After using GitJournal for some time it eventually failed to push its changes while it would still pull external changes. Taking a closer look it turns out that some of the tree objects are not properly sorted:

git fsck
error in tree 00c6beca0b3ac8c41dca091ddba67065be64d90f: treeNotSorted: not properly sorted
[...]
error in tree def8c7023c6f9681da412e6202bd8e085dd5b778: treeNotSorted: not properly sorted

I can't spot an obvious cause but it's visible that the lists (like `git ls-tree 00c6b`) consist of a correctly sorted list followed by a second correctly sorted list, so depending on the sort algorithm it might be an off-by-one error? Also, if you use a library for git matters, it's probably an upstream issue.