The bug is found under the traffic reports, in the "Rank and Traffic Stats" section, under the graph where it lists out the keywords you rank for and how valuable they are on a CPC basis.
In that section, instead of just listing out keywords, Bing is also listing out weird strings of numbers. Here is a picture from my Bing Webmaster Tools account:
Bing has yet to reply with any information about this bug.
Forum discussion at Bing Webmaster Help.
Update: Duane from Bing said this was fixed recently:
We had a database error that has now been fixed. Fresh data populated over the weekend, so now reports should all be back to normal and showing accurately.