What operations or methods differ in that they lead to a change in the system state upon re-execution, rather than preserving the previous result?

Operations or methods that change the system state upon repeated execution, rather than just returning a result, are called side effects.

Examples:

• Writing or modifying data in a database.
• Sending HTTP requests that change server state.
• Writing to a file or modifying the filesystem.
• Changing global or static variables.
• Calling functions that modify object or system state.

In testing, it's important to distinguish pure functions (without side effects) from functions with side effects, as the latter require special handling for verification and isolation.