Generally, if you make a mistake in a DIESEL expression, what went wrong will be obvious. Depending on the nature of the error, DIESEL embeds an error indication in the output stream.
| Error message | Description |
| $? | Syntax error, usually a missing right parenthesis or a runaway string |
| $(func,??) | Incorrect arguments to func |
| $(func)?? | Unknown function func |
| $(++) | Output string too long, evaluation truncated |