When creating an invoice manually the following can occur:
1. Enter a description in the first line item
2. Enter an amount in the first line item
3. Do not click Save, instead click Publish
4. The invoice is published, but no line items are saved
When the page refreshes after publishing the invoice, you can see that the draft invoice was created, but the line item entered is not saved. To correct this, you must remember to save the line items before publishing the invoice.
Which brings me to my feature request: when publishing an invoice, I'd like for WHMCS to save the invoice line items automatically so the admin does not have to remember to save the line items prior to publishing.