When going to the 'Disc IDs' tab of a release. Every medium is listed twice. The disc IDs, if present, are only shown along with the 'second set' of media.
Additionally, when on the 'Disc IDs' page, under 'Release information' on the right side of the screen, format also reports a double amount of mediums, e.g. Format: 4xCD, when there are only 2 discs. On the main release page, however, the format given is correct (i.e. 2xCD in this example).

Examples:
http://musicbrainz.org/release/7537069d-c82b-43a7-8b22-c9401ecfdb82/discids

This bug also happens when the format is not 'CD'. In that case it (probably) only manifests itself in the 'Format: xxx' string.

Example of vinyl reporting a double amount of mediums on the 'Disc IDs' page:
http://musicbrainz.org/release/88066d9e-5292-4463-af9b-894cb7a70d99/discids