While different nodes have different errors, all nodes (except plots) have input errors/warnings.
Nodes may display an error or a warning icon in the top right corner:
Hovering over the icon will display a diagostic message.
Errors are critical and indicate that the node is unable to calculate its outputs, and the simulation is not runnable.
Warnings are non-critical and indicate possible oversights that may lead to surprising behaviour.
Nodes that allow writing a math-expression have errors looking like "parse error matching struct...". These errors indicate an invalid expression. For a reference of allowed Expressions, see the Expressions-page.