Bad things? I think they should have been a bit more specific for you
You should avoid putting a hard drive (especially one with the OS or programs) on the same IDE channel as a CD/DVD drive. Reason being that the cable will now run only as fast as the slowest component, which is the CD/DVD drive. So architectually, you should always avoid putting a hard drive and CD/DVD drive on the same channel. If you have another IDE channel put the hard drive on that, and leave the CD/DVD to its own channel.
Nothing 'bad' will happen, it is perfectly fine. Nothing is going to happen with this configuration, it is just that your hard drive will now transfer data much slower to the MB because of the DVD drive. Make sense?