Debugging spells is just as much a dark art as spell crafting itself. When I was a young apprentice we didn't have as sophisticated tools as you do now; you had to make sure you noted down your intermediate runes correctly and use those symbols to divine some meaning from the ashes of your failed spell. One time I mixed up my notes with the symbols of a different spell and when I sprinkled the ashes on the stack I was stuck speaking in tounges for a week.

These days of course you can summon a lesser demon to freeze your spell and ask it about the state, but the demons can be tricky and it's easy for novices to make a mistake and allow the demon to run amok - makes a real mess of the lab.