Each year, consumers return billions of new products to sellers. Despite reports that once returned products are often discarded without ever being used, surprisingly little research has examined the environmental impacts of returns from a full lifecycle perspective. Building on a unique dataset covering over 630k returned apparel items, we map the flow of returned products under best and conventional management practices, and illustrate the full lifecycle impacts associated with returns using two case studies. Our results suggest that 23%-46% of new products returned never reach another consumer. Moreover, we find that the GHG emissions associated with the production and distribution of returned products that ultimately go unused can be 16 times higher than all post-return transport, packaging, and processing emissions combined. Our findings suggest that the environmental performance eCommerce, where return rates are exceptionally high, may be systematically underestimated, and highlight the urgent need to promote circular returns management practices.