Steps:
1) Run PL/SQL Developer - connect to schema with objects to compare.
2) Run 'Compare User Objects' from Menu Tools.
3) Pick up 'Target Session' schema to compare.
4) Click 'Compare'.
5) While Compare is running switch from PL/SQL Dev to any other Windows App - in my case MS Outlook.
6) Go back to PL/SQL Dev - Compare hangs - progress bar is frozen.
Note: I tried Compare with 'small' schemas - progress bar became frozen, however compare had finished. Maybe this is display refresh problem? This is my guess only.