The InvalidInputException is thrown by many plugins that provide
additional validators, like for example to make a field mandatory
during a transition or to force the user to modify a fields content.

When the screen of such a transition has also a user picker field,
that might be the assignee field itself, or any additional custom field
of this type, whatever the user enters will be lost, once the plugin
throws the exception during validation.

For example force the user to change the Fix Version/s field.
Assuming he only modified the Assignee, but forgot the
version. See attached before and after screenshots.

In that case JIRA, will interrupt the workflow transition, and the user
picker field it's content will become invalid. Sure the user will see
the red message below the Fix Version/s field, and corrects the
input. But he won't see that the Assignee field contents changed
too, into a simple text input, non validated by JIRA, and thus when
tying again to complete the transition, the assignee input will get
lost.