Use of Manual Focus lenses on autofocus bodies.

Hello, i'm half sure this is the right forum for this question, if nothing else it gets more traffic. I'm buying an ai/s 300mm 2.8 lens soon. I remeber reading once that a guy put a Nikon MF lens on his F5, but his auto focus was still 'on' so when he half depressed the shutter to check for exposure, it messed up the f5's autofocus motor. Is this a real event? Do I always need to have my D2h in 'M' mode when using MF lenses? Or is this just hearsay? Thanks.