If a Wizard fails to equal a spell's casting value, or the total of the natural dice scores is 1 or 2, not only is the spell not cast, but the Wizard breaks his concentration and is unable to cast spells for the rest of the Magic phase.
For example, a Level 4 Wizard attempts to cast The Flock of Doom (casting value 5+). Assuming that his innate magical ability (the +4 bonus for being a Level 4 Wizard) is sufficient to ensure that the spell is cast, the controlling player chooses to use a single D6 in the casting attempt. Alas, the dice scores a 2 and, even though the casting total (2+4=6) is higher than the spell's casting value (5+), the spell fails. The Wizard will not be able to cast any further spells that turn.