Now what kind of error caused this?

Early this morning, sighted in the river Avon, close to my office in my home town of Bath was what appeared to be a stranded naval vessel.

Bath is home to several Royal Navy departments

including Submarine design, however none of their designs have previously been seen outside the confines of the secure MoD establishments.

Curious onlookers gazed from Pulteney bridge and wondered what kind of navigation data error caused this. 

Or maybe it was a date er error ;)