Here is my favourite drawing illustrating one of the problems with Subversion:

                      |
                      | file p.txt contains "X"
                      |
 +<---(branch)--------+
 |                    |
 * file p.txt renamed |
 | to f/p.txt, which  |
 | contains "X"       |
 |                    * the content of file p.txt is updated to
 |                    | contain "Y"
 |                    |
 |                    |
 +----(merge)-------->+ file p.txt is deleted and file f/p.txt is created
                      | with the content "X"  
                      |
                      | BUT expected/wanted content is: "Y" (or at least a conflict)
                      V