Since switching to W3 Cache, I was getting constant 502 proxy errors within my admin area, and it conflicts with transactions on the eCommerce section of the site, not allowing for a transaction to finish processing.
I've played around with settings and it worked fine for a while, then started acting up again recently. Since then, I have had caching off.
Using WordPress 3.1.1, genesis custom theme, jigoshop ecommerce plugin.