Circular dependencies in Node.js modules can lead to unexpected behavior, such as incomplete module exports or errors during runtime. These happen when two or more modules import each other directly or indirectly, creating a loop. To fix this, identify the cycle and refactor shared logic into a separate module that both dependent modules can import without depending on each other. Another approach is to use dynamic `import()` statements, which delay module loading until needed, breaking the circular reference chain. Using tools like `madge` to visualize module dependencies can help you identify and resolve circular dependencies before they cause problems.