Magical Build-up, or Magical Back-Up, is a condition where a fairy explodes and turns into fairy dust. It only happens if the fairy cannot grant wishes to their godchild (as shown in "The Same Game") or if the fairies do not perform wishes for a certain amount of time (as shown in "Fairy Idol").
If a fairy is not able to grant wishes to their Godchild, the magic will begin to build up inside of them until magical build-up occurs. The length of not granting wishes depends on the fairy's size as well as the overall conditions that are occurring for the godchild and/or the world (for example, in "The Same Game", the smaller fairies experienced Magical Build-up only minutes after losing their godchildren and being unable to find their godkids due to everyone looking exactly the same. While in "Fairy Idol", Norm (who is larger) took a full two weeks to experience it. Unlike in "The Same Game", Cosmo and Wanda get Magical Build-up after 24 hours of not granting a wish, which might have possibly been because there was no major crisis for Fairykind on finding their godchildren). Once Magical Buildup occurs the victim will begin to burst into sweat and shakes. Followed by inflation. The victim will blow up larger and larger until they burst into a pile of magic or confetti.
The only known cure is to grant a wish before or during Magical Buildup. Afterwards the only way is to be wished by someone else back to normal.