Test suites and test cases. Tests are usually written first and drive the development. When encountering a non existing message, the debugger pops up and asks you if you want to create the message.

Usually, this makes one think: "ha, this is not right" and then goes back on the execution stack and either fixes the test and continues or refactors something etc.

But in a dynamic language, tests are required to avoid regressions.

Programming as teaching goes both ways, programmer is teaching Smalltalk, Smalltalk helps in revealing thinking flaws to programmer. Loop continues.