What does a spell going "haywire" mean?

Haywire Chance is described as

"Haywire Chance affects how likely your spells are to trigger a highly unstable resonance cascade event."

but what does it mean practically?

Solution 1:

Your Haywire Chance stat is the chance you have of trigger a haywire effect when casting a spell, this is basically the spell equivalent of a critical hit.

However, haywire is broken in version 1.0.3 and doesn't work as intended. We can see this from the blog post describing the changes to be expected in 1.0.4:

Bugs, Actual Bugs:

• Haywire not triggering, except when it absolutely shouldn’t. (Vampirism, traps, everything but your spellcasting.)

Solution 2:

To be specific, Haywire does exactly one thing: it ignores enemy Magic Resistance (which at low levels is a huge effect in and of itself). At one point it also doubled Magic Power for that spell, but that turned out to be too huge, so it was dropped. Ignoring Magic Resistance is plenty strong on the lower floors. :)