Accidentally took a zantac (ranitidine) that is exactly 1 year past expiration date...how bad is that?

No problem: No problem, but take the rest of the bottle and flush the tablets down the toilet so history doesn't repeat itself.

Probably not bad: There is a reasonable chance it won't do anything to help your heartburn, but otherwise probably not going to do any harm to take an over the counter (otc) past its expiration date. That being said, try not do that again! throw out the package to prevent some mistakes. Routinely going through medicine cabinets to remove old and expired drugs is also always a good idea.