On Oracle, we have a size limitation for issue Description and Comment fields of 4000 characters.

So, if you add a description or a comment that is greater than that, you get an error:

Exception: com.opensymphony.workflow.InternalWorkflowException: An unknown exception occured while executing FunctionProvider: com.atlassian.jira.workflow.function.issue.UpdateIssueFieldFieldFunction
at com.opensymphony.workflow.AbstractWorkflow.executeFunction(AbstractWorkflow.java:934)
at com.opensymphony.workflow.AbstractWorkflow.transitionWorkflow(AbstractWorkflow.java:1107)
at com.opensymphony.workflow.AbstractWorkflow.initialize(AbstractWorkflow.java:366)
at com.atlassian.jira.issue.IssueUtils.createIssue(IssueUtils.java:80)
at com.atlassian.jira.issue.IssueUtils.createIssue(IssueUtils.java:65)

The problem here is that there is no hint as to what caused the error. It would be better to be able to give an error:

"Your comment is too large. Consider creating an attachment."

Its practically impossible from the Entity Engine exception, to programmatically work out what happened (even the error message depends on what version of oracle)
The only other thing to do is to make it part of the Web Action validation - thereby forcing a limit on everyone.

This would actually be a good thing, because they might otherwise end up hitting this limitation when changing databases (as we did). Especially since HSQLDB in the stand-alone version doesnt have any limit....