Along with the SimpleCart hotfix for FormIt 4.2 compatibility, an update for the Mollie gateway for SimpleCart is also available today.
v1.2.2 of the gateway started marking orders as awaiting an asynchronous payment confirmation, to support payment gateways that are not instant, but it failed to unmark the order as such when a transaction came back as failed or cancelled. This would cause the checkout to get stuck on the “Pending transaction confirmation” view, rather than showing the payment failed.