That exact same thing happened to me for the first time yesterday (regarding the spider spawning incident - I haven’t missed any 4 of a kind extra turns).
It’s the weirdest thing. The spider summoning hero class is bugged and causing weird issues.
And to the guy directly above me… Relax. Stop freaking out. He clearly thought something weird was going on. Not everyone knows about bugs they haven’t experienced. Don’t be so rude.
The first time I experienced it I had a “WTF” reaction, I just didn’t post about it. It’s a really weird and interesting battle when the bugs happened. Seriously though, don’t be a jerk. Not everyone is as awesome as you apparently are