Resisting nage is a very tricky thing. Too often it is merely the mechanism whereby one aikidoka asserts their "superiority" over another. Resistance also usually takes significant advantage of the fact that uke knows in advance what nage is going to do. Even very junior aikidoka can defeat a senior aikidoka's technique if they know ahead of time what he/she is going to do. Resistance also creates tension and a certain inflexibility of mind.
In my experience, there have been mighty few times when resistance from my uke has truly been instructive. Sometimes, it has been downright injurious. I have torn the tendons of a few uke who suddenly tried to halt or escape my technique. And it has been, in my experience, the higher-ranked aikidoka who seem the most prone to misusing resistance - usually because there is a strong attitude of "only my way" in these aikidoka and/or a need to justify their rank in the eyes of those equal or junior to them.
I don't usually actively resist other aikidoka in a contradictory way. Generally, when they move wrong they don't take my balance, or they return my posture and balance to me. When this happens, I naturally feel heavier and harder to move. Even very new students can feel this. This kind of "resistance," however, doesn't leave nage feeling like their technique was defeated. They don't feel like uke has interfered with what they were attempting to do, but rather that uke simply responded to what they were doing. I find this induces nage to think, not about what a hard time uke is giving them, but about the effect of their actions upon uke. Instead of fracturing nage's focus, as contradictory resistance usually does, the kind of "resistance" I give usually encourages nage to focus more intently upon the form and effect of their motions.