Putting 8-sprocket cassette on a 18-speed bike I have an 18 speed and want to swap the wheels out, but the new rear wheel has an 8-sprocket cassette and the one on the bike has a 6-sprocket . I plan on swapping them but just for fun I tested it out without swapping them and the 8-sprocket rides just fine even though two of the sprockets will never be used. 

Would it cause problems to ride with the 8-sprocket or can I just leave it on and won't experience any differences?
As long as you're running a narrow six speed chain, there shouldn't be any issues as the width on those is the same as a seven/eight speed chain. You may want to adjust the rear derailleur and you can probably hit all eight speeds on your cluster. If you have index shifting you will need an eight speed shifter.