The East Lothian men had come out on top in their first ten league encounters this term, but number 11 was to prove a game too far for them.
They headed down to the Borders and returned with a 17-15 loss against a St Boswells side, who are still undefeated.
Murray West and Angus Elder scored NB’s tries while Liam West kicked a penalty and a conversion, but they could only gain a losing bonus point.
It means they are still top of the table, but St Boswells are now just six points behind with two games in hand.
Murrayfield Wanderers are in third place after they went to Forrester and won 37-3.
Linlithgow are on the same number of points as Wandies, but were defeated 17-11 at Hawick Harlequins at the weekend.
Dunbar were comfortable 33-0 winners over Hawick Linden at home, while Dalkeith lost out 15-13 at home to Duns.
Penicuik are still out on top in BT East Two after they edged out Leith 10-0.
Inverleith got the better of Liberton 15-3 while Ross High came back from Langholm with only a losing bonus point from an 8-6 defeat.
Edinburgh University Medics pushed BT East Three leaders Earlston close, but lost 8-6 while RDVC lost 29-15 to Walkerburn and Queensferry lost 55-12 to Gala YM.