Ok so I've only started using WHMCS a few days ago and I've already run into a major "bug" if you can call it that - it's more of a really badly designed "feature", namely the "mass payment of invoices".
So I had a customer with 2 due invoices, let's call them invoice1 ($5) and invoice2 ($10).
The client logged into WHMCS and selected both invoices and selected "Pay All". But the client changed his mind / got distracted and didn't actually pay there and then.
What happened? Well what happened is that, WHMCS created a new invoice3 ($15) that is ONLY visible on admin-side. 2 days later client paid invoice1 & 2 via bank transfer and I marked those 2 invoices as PAID - client received confirmation via email and everyone's happy.
Yet, there was still invoice3 ($15) in the admin backend which was marked as UNPAID - wtf? (I didn't see it)
Not only is there invoice3 - but customer gets a reminder about it. HA! They can't even SEE that invoice on the client side!!!! And the 2 invoices it refers to are marked PAID!!!
This is a really really reeeeeeeeally stupid way of "mass payment" that only causes inconsistency and confusion, IMHO.
In my opinion the only acceptable way of "Mass Payment" is to allow customers to pay multiple invoices (ie. "Pay Selected" or "Pay All") by using an instant payment method ie. CC. If they don't pay, no harm no foul. If they pay, the selected/all invoices are marked PAID. Simply. No new invoices created or necessary and no inconsistency created.