Twice during the sahreeli event the game has frozen while casting dark.petition. It is not every time, but I have screen shots.
Was playing sakrain the mad on both occasions.
If you are using prized amalgam its a know bug.
Otherwise a list of creatures you use in the deck could help.
Akoun firebird
Runaway carriage
Breakneck rider
Village messenger
Oh well is not easy.
Only one we can exclude is runaway carriage.
Firebird cause some issues to me and having a self resurrect skill it could be the source.
However strange behaviors has been reported with transformation card especially if those card did transform before dying/being bounced.
It could be any of the three.