run-webkit-tests's pretty diff currently does not support non-ASCII characters. As a result, non-ASCII characters such as those used to test RTL texts are corrupted in the expected results when the test uses dumpAsText. Try editing/text-iterator/rtl-selection-crash.html for example.
FYI: diffs have no encoding and must be treated as binary.
Actually, I think that expected results are always UTF-8.
Just wasted some time due to this; haven't ever looked at pretty diff, how hard is this to fix?