The Spiegel report, as the Associated Press notes, states that BND accidentally recorded a call Kerry made on a satellite phone in 2013, and recorded another call from Clinton in 2012.
According to the report, BND destroyed recordings of the call immediately afterwards, and neither Kerry nor Clinton were the intended targets of its surveillance operation. In the case of Clinton's call, BND meant to target a terror suspect's communications, but Clinton's call was on the same frequency as the suspect in question.
Der Spiegel didn't name sources in its report, the AP notes, but the publication has reported extensively on intelligence agencies' use of digital surveillance in recent years.
To say that this alleged incident puts German officials in an awkward spot would be quite the understatement, especially considering how, in October of last year, word leaked out that the National Security Agency had spied on 35 world leaders, including German Chancellor Angela Merkel.