I just noticed a case where "git blame -M" ran on a file containing only line reorderings in the last commit would still report some files as being introduced in the reordering commit. Reproduced with Debian squeeze's 1.7.2.3, as well as with 1.7.4.rc2.
Sample case is available from https://alioth.debian.org/anonscm/git/omaha/omaha.git, commit 369b92da.
"git blame -L325,354 -M 369b92da README" shows the reordered lines, where lies 332, 333, 338 are reported new in this commit.
It does not seem to be related to the -M threshold.