How would you depict errors and alternative scenarios in a Sequence Diagram?

In a Sequence Diagram, alternative fragments (alt fragments) and exception handling are used to display errors and alternative scenarios.

• alt fragment allows showing different execution paths, such as successful and error scenarios. Inside an alt, multiple options with conditions can be specified.
• For errors, a separate alternative block with a condition, such as [error] or [exception], is usually highlighted.

Example:

alt Successful scenario
    User -> System: Request data
    System -> User: Data
else Error
    User -> System: Request data
    System -> User: Error message
end

Thus, the diagram shows different possible developments, including errors and alternative paths.