Subversion does not detect moves as other version control system does. This could be a problem when you rename or move a file not using the appropriate subversion command.
For example, if you rename a file named "foo" to "bar" without using the Subversion command, it shows "foo" as "missing"and bar as "non-versioned":

$ svn status
?       bar
!       foo

While TortoiseSVN has it's own "Repair move..." feature, from the command line perspective I had had to fix this manually until I wrote a simple script that handles this situation.
It works like that:

$ svn-repair-move foo bar
$ svn status
D       foo
A  +    bar