Randomly started spectating a friend's Arena game and got to see one of the worst plays of all time:
Some rando, Priest, vs. our hero, Paladin
Rando is at 1 health and is losing on board with 10 mana.
Rando: Topdecks and plays Deathwing, proccing Dark Cultist for a 12/15 Deathwing on board
Hero: Plays Argent Squire, Unstable Ghoul, makes guy
Rando: Kills Unstable Ghoul, plays Deathlord, heals to 3
Hero: Plays Ancient Mage, makes guy
Rando: Clears board with minions, plays Leper Gnome, heals to 5
Hero: Plays Fen Creeper, makes guy
Rando: Plays Shadow Word: Pain on Fen Creeper, clears board, hits face with Deathwing (Hero now at 11 health), heals to 7
Hero: Plays Fel Reaver, makes guy
Rando: Plays Dancing Swords (hero discards Guardian of Kings, Sludge Belcher, Loot Hoarder to Fel Reaver), attacks Deathlord into Fel Reaver, summoning Lord of the Arena, attacks Deathwing into Lord of the Arena, heals to 9
Hero: Attacks with Fel Reaver and guy for 9 damage and exact lethal.
This is why you don't mess around with RNG-based BM.