I had a spiral effect on a mega tree....I deleted the effect, rendered but the effect was still present. There is no effect showing, but when you toggle the model you can see the effect. The only way to remove it is to add an "off" to the mega tree.
BTW: this example has a wholehouse ramp down above the mega tree model, with a spiral on the mega tree...delete the mega tree effect doesn't remove it from the wholehouse display....Yes I did rendered several times...