Can a western blot test be wrong?

Tiny: Every test has a margin of error, albeit this one is microscopically small, and every operator has a risk of performance error, again tiny as WB is mostly automated. Talk to your Dr about the result and any risk of error, as it is most likely accurate.

Lab tests: Any laboratory test has both false negative and false positive results possible. Generally the Western Blot is so specific that it is rarely incorrect, but can be. If there is any question, repeating the test is the way to go. You don't say what illness was being tested for, but there are often alternative tests that can be applied and labs are only of value if taken in a clinical context.