In SourceTree Mac 1.6.0b1 (but also in 1.5.8)
On the Repository Window, File status File Status View, with a Git repository and staging enabled, the differences view (shows the contents of a selected file) displays mangled line numbers for some file types.
I first noticed this with a CSV file, but then observed it with a file with no extension (like README).
Here's a video that demonstrates the problem I'm seeing:
http://youtu.be/qWgUz89uHuY
And here's a screen shot:

Seems most likely that this is user error of some sort, but I can't see how. Any ideas?