My mouse is a bit buggy, and sometimes it double clicks unintentionally. Today, I played with a deck that had steal and lightning in it, as well as supernovas.
When I clicked steal/lightning, and a supernova was underneath the card, and my mouse glitched, it triggered the supernova while dispaying targets for the spell I intended to use. When I pressed cancel, the spell got replaced by a supernova.
This has happened on three unintentional occasions, but I managed to manually reproduce it once as well.
I'm not sure if this is a "clicking too fast bug", but this can seriously outbalance a game when abused.
I haven't tested if this can be done with any targeting spell+instant, but I know that the combos above worked.
