I created a simple Cocoa app (Mac 64bit) in Xcode, and in it I created a string object, and then outputted the contents of the string in a NSLog statement.
Then I decided to see if I could modify the contents of the binary (exe) in the .app directory of the application. I used 0xED to change, This is my string. to This is my new string. I did this by typing the word new in the right portion of the 0xED editor.
Finally, I saved the file, then tried to launch it, but it appears to crash. The crash report appears somewhat cryptic to me, so I am not exactly sure why the app is crashing.