When you create a new invoice (or quote or PO), it can happen that the system doesn't give the next logical number to this invoice, but instead a number that has already been used for another invoice.
The problem comes from the issue dates of your invoices.
LSP.expert finds the latest invoice created based on its issue date, and increments that number for the next invoice. So if you modified the issue date of an invoice after creating it, it might happen that the system uses the wrong number. The same happens if you modify an invoice number.
In the example above, let's say you changed the date of issue of your invoice #042 (the one at the bottom) for some reason. The invoice with the latest date of issue is thus #041. You can see that by sorting you list based on the date of issue. When you create your next invoice, the system uses #42 again because it takes into account the latest invoice created at the latest date of issue (in this case #41).
To correct this, you have to sort your list of invoices based on the issue date (click on the little arrow next to the 'Date of issue' column) and update the document numbers or the dates of issue correctly. In the example above, you can thus rename your second invoice #42 into #43, or change the date of issue of your first invoice #42.