Back in 2000, Jacob Nielsen proposed that elaborate usability tests are a waste of resources. It's often discussed and sometimes questioned advice, but Nielsen's test data on testing is hard to dispute.
“The best results come from testing no more than five users and running as many small tests as you can afford,” he found. But, if you want to to discover all the usability problems, “you need to test with at least 15 users.” That said, “The ultimate user experience is improved much more by 3 studies with 5 users each than by a single monster study with 15 users.”
This more recent article from Nielsen is worth a read too if you are introducing changes to a user interface. “Any time you release a new user interface design, you'll get complaints,” he warns.
That's simply because “users don't like to learn different ways of doing things,” and there's nothing you can do to change it! 😡
What you can do is reduce the amount of change you inflict on users at one time.