bikerboyd

I have an 8 speed driveline on my mtb right now. What kind of problems might I encounter if I put a 9-speed derailler on the bike?

well biked

No problems, particularly if you're talking about the rear derailler. Derailleurs don't have "speeds," the "speeds" are in the shifters and cassette. When a rear derailleur is referred to as "nine speed," "eight speed," etc., it's marketing, it reflects whatever the current componentry was at that time. For nine speed mtb stuff, of course, "nine speed" is the current stuff for most of the groups-

MtbVA

No problems at all.