User blog comment:Linathan/Squad Spotlight & Guide: Trial EX6 - Breaking Barriers/@comment-26938351-20160428153851/@comment-26467509-20160506224143

I'm not sure. I used 3 BB's at all times every turn when I did this with no issues. Never got leaderskill locked/nuked/bb wiped unless I wasn't paying attention and activated four SBBs. Realistically I think you should have been fine using a UBB and two SBBs on Grahden's 15% mark - maybe the game had a small hiccup?