Mosu has released MKVToolNix 1.4.1, primarily a bugfix version. Here’s a partial bugfix log:

* mkvmerge: bug fix: AC3 detection was broken in rare cases. * mmg: bug fix: If the TEMP environment variable contains spaces then the calls to mkvmerge when adding files failed. * mkvmerge: bug fix: Extracting the FPS from some AVC MP4 files did not work. * mkvmerge: bug fix: Appending + splitting was segfaulting if used together and at least one split occured after a track has been appended. * mkvmerge: Added more descriptive error messages if two tracks cannot be concatenated because “their parameters do not match”. * mkvmerge: bug fix: A failing call to posix_fadvise will only turn its usage off for that one file and not abort mkvmerge completely.

... and many, many more. You can get the goods here.