When porting an app from Windows to Mac, a common mistake is to map Alt to Option. This is tempting, since it apparently frees the Command (aka Apple) key to be used for normal Mac commands.
Unfortunately, this is incorrect. Option is used on the Mac to signal non-standard keystrokes and key combos. For instance, to type an accented e (?), you type option-e-e. The first option-e puts a plain accent; the second e turns it into an "eacute" or accented e.
The bottom line is, in IDEA, option-keystrokes are often intercepted at a very low level. This leaves junk around in the text editor, and often erases the very text you were trying to apply a key command to, or leaves some junk around in a file. For instance, try doing control-option-leftarrow to go "back"; when you return to the original buffer you will see some junk.
The solution is for porters to just pretend that the option key doesn't exist. Please go through the OS X keymap and either rename Meta to Alt, or remap all Alt-using key maps to use Meta. The point is -- don't use option!
(Yes, many Mac menus use Command-Option combos. This is acceptable because of the presence of Command, since it signals that it isn't a key input option.)