Python's interpreter has several unexpected behaviors that developers should be aware of when writing code. Mutable default arguments can lead to unintended behavior, as the default value is evaluated only once and retains its state between function calls. Integer caching means that integers are objects created at the start of the interpreter, which can affect comparisons using the identity operator. Late binding closures occur when a lambda function captures a variable's value during loop iteration instead of its current value. String interning stores small strings only once, but larger strings are stored separately. The += operator behaves differently with mutable and immutable types, sometimes modifying original objects in place and other times creating new ones. By understanding these quirks, developers can write more reliable and maintainable code using tools like SonarLint and SonarQube to help catch mistakes.