So, when 1 or less characters in the first line of a DAT, this results in the first line being omitted. It seems like the first line is still in the DATA of the DAT, but editing it afterwards results in the line missing.
THis also happens when looking at a file twice from two panes, where one will display the original, another one with the omiited first line.
In most cases this is not a huge issue as the omitting only happens if you again manually touch the DAT, in which case you can also simply put in a second newline at the beginning.
