When applying multiple tags to one post or page, then displaying a custom field on that tag's archive page, the custom field values for both the tag page and the other tag's custom field values display on the page. Obviously, only the custom field value for the current tag should appear.
To illustrate the problem less abstractly:
Tag 1 has this custom field value
Tag 2 has this custom field value
I tag a post/page with both Tag 1 and Tag 2. On Tag 1's tag.php page, where I have the custom field values displaying, both custval1 and custval2 display, instead of just custval1. This appears to be a bug, though I'm still looking into why it might be doing this.