If Rakan brings himself under sufficient mana to cast his E, Battle Dance, he is unable to recast the ability despite it being free. That is to say if he casts Battle Dance and brings himself under 60/70/80/90/100 mana, or if he casts Battle Dance and any other of his spells that result in him having less than that much mana, he cannot recast Battle Dance for free.
Maritzol2 votesYes, Bug also happened to me as described above.
Σmsi2 votesSame with me, it cost me so many saves because I couldn't recast 😭
AHarmlessTaco1 votesUhhhhh TBH Im usually the one that dies because of it. XD
Darabasi1 votesThis is a really serious issue due to Rakan players always going all in and only having the second dash as their lifeline. Since its stated "at no cost", it should be castable below the costs of the first cast. 
AHarmlessTaco1 votesYeah, it was working as intended before the patch, not sure what happened. It is a fairly substantial problem though, it usually comes up at least once each game, if not 2-3 times (rare).