This problem stems from the fact that GTN mail has the exact same subject for the initial mail with 0 credits and the following mail with credits attached. So as you're going through a lot of no-credit mails and deleting them, you come across one with credits, and bam, deleted. Granted, I could take more time reading mail and probably avoid it, but it seems like obvious functionality as almost nobody intends to delete mail with items or credits attached.