Some sections are Linked, such as double graded seminar sections, some labs with lectures, and drill sections with the class, etc. Otherwise, it will appear in the Registration Errors box with the Link Error message. It actually is 1.0 credit, but for Pre-Enrollment purposes, in order to make it not count toward toward your 4.5 credit limit, we have temporarily zeroed it out. It will be 1.0 credits in reality. The computer worries about that for you, but you do not need to worry.
Nothing is prevented by time conflict.
You can even pre-enroll for a true time conflict that you do not intend to keep. The old PRE is not available. Don't use the BACK button on your browser. If this doesn't solve your problem, try closing your browser completely and trying again, or even trying another computer. Some problems may be due to things we can fix on Monday, so expect that many problems which arise over the weekend will be solved by Monday afternoon.