MS GP: Why did I get remittance printed automatically?

Remittance gets automatically printed if a check amount is equal to zero or you pay more than 12 vouchers with one check. The second part is easy to understand. What about checks with zero amounts? You can have this situation if Automatically Apply Existing Unapplied boxes checked in the Select Payables Checks window and unapplied amount is equal or higher than voucher amount. In this case during the process of building a check batch GP will automatically apply all unapplied payments, credit memos, returns and as a result some vendors may have a net amount of zero. If this happens the system will create a remittance record that will begin as "REMITxxxxxxx" and print a remittance to show the vendor what documents are applied. A remittance is not a check and it doesn't get posted to Bank Rec. The next REMIT number is automatically incremented and pulled from the PM Key Master table (PM00400).

To prevent this from happening you can unmark Automatically Apply Existing Unapplied boxes and then apply document manually or if a check batch is already built use the Edit Check window to exclude the applied documents in this check batch.

More info:
KB:861276 - How payments that have amounts of zero are treated in Payables Management in Microsoft Dynamics GP
https://mbs2.microsoft.com/Knowledgebase/kbdisplay.aspx?scid=kb,en-us,861276&wa=wsignin1.0