I recently had a user call and ask why he couldn't copy and paste properly inside of SAP, it was working properly outside of SAP. So, I took a look at his problem and saw that everytime he highlighted some text it would automatically unhighlight itself. After looking through some of his settings I noticed that he had "quick cut and paste" enabled.
To change this setting you must click the "Customize Local Layout" button on the toolbar, and the check or uncheck "Quick Cut and Paste."
This copies text as soon as you highlight it and helps with areas where you would normally have to use Ctrl-y prior to copying. But, it causes some unexpected behavior in areas where you would normally not have to use Ctrl-y. I recomend that if you are in an area of the system where Ctrl-y is necessary a lot that you turn it on, but turn it back off when you are done.