Why would failure cause [i]everything[/i] to explode rather than just not work is the big question

Like, unless you're [i]trying[/i] to make something with a somewhat similar effect where 'it isn't working as intended' could be 'destructive release of extra magic in a way that explodes', I don't see why they'd go boom. 

uh, wall of text incoming, because i'm copying the magic information stored elsewhere:

[quote]The TL;DR version is: every living creature generates mana. Sapient races generate more, and individuals amongst those races can have such an excess that it can be moved through certain patterns to effect spells. Dwarfs, for example, never have a sufficient excess (but are sensitive enough to its flows to design some incredibly intricate runework), and almost every elf does. Humans are somewhere in the middle, and Hundi are odd--Hundi mages are rare, but when they do crop up they're often very powerful.

The act of spellcasting is therefore a matter of taking sufficient mana for a given effect and forcing it into the proscribed pattern. How this is done often shows the type of magical education a particular person has received: the various magical colleges like to take an analytical approach, and their spells are strongly formulaic, even if the resulting phrasing doesn't make sense in normal speech: they focus on creating a general language to describe the flows of mana. Similarly, when conducting rituals, circles outlined in chalk or similar tend to be heavily geometric. Self-trained or hedge-witch trained mages tend to have far more personalised spells, or use other methods of casting entirely--singing, or dancing to maintain a long-term effect, etc.

The purpose of such guidance is to associate the movement of something as ephemeral and invisible as mana with material actions, making it easier to control. With practice and experience, the spellcaster can cut down on the necessary ceremony (at the cost of some efficiency). It is possible to cast magic directly through willpower alone, but this requires either a vast reservoir of mana to pull from, or means honing a very specific talent to the point of being instinctive.

(An analogy could be made that the pattern required is like filling a series of channels in order. With a spell and enough practice, ONLY the correct channels are filled, whilst skipping steps or less practice overflows in places--normally just wasting mana. Simply willing magic to happen is akin to flooding the entire plain, regardless of channels, and taking the resulting backlash into your own soul; it's very dangerous if you can't cushion it with even more mana, on top of being orders of magnitude more wasteful) 

Runes are the physical depiction of these patterns, adapted to the medium (from its material to its very shape and function) they are being incorporated into to facilitate the flow of mana from whatever the source is. Unlike ritual casting, this isn't some method of conceptualising the pattern to direct mana from yourself (or channelled through yourself); it is the pattern... in dozens, if not hundreds or thousands, of tiny symbols, carefully attuned to the object in question: such accuracy is impractical for active spellcasters.

Due to their own low mana capacity, dwarfs would prefer external sources of crystallised mana, while the magically gifted can power them directly. The third case, of blessed items keeping to the limits of magic, are powered by the deity in question.

The origin of these patterns is twofold: many were made part of the world during its creation and by the actions of the gods in earlier times. This encompasses essentially all elemental manipulation, for instance. Other ideas were reified by enormous effort of archmages over the years, branching off of something that's 'close enough' to exist; many subtle variations on emotional effects or extremely specific healing spells came around in this way. A third category is to use mana in place of a similar enough process--the action of the soul to sustain life, which leads to every form of undeath but also beneficial necromancy for medical purposes (where the relationship between the physical and metaphysical is too damaged).

'True' Witchcraft, by contrast--although something that is only known generally to magical scholars from historical accounts--doesn't follow any established pattern. No matter how much mana you have, you could never replicate them through the medium of spellcasting. A mage with enough practice can teleport limited by their mana for range; the correct witch can place two places side by side or overlapping with a distance-independent amount of effort. The exact nature of this is kept a closely guarded secret by the few practitioners who know the why rather than simply how. A noted similarity of some recorded acts of true witchcraft with the abilities of demons has not, however, yielded a satisfactory working explanation to replicate it.

Mana in sufficient quantities, decoupled from life, can crystallise into a self-perpetuating form. The 'common' version would be magical crystals and gemstones, either intentionally imbued into such for their own stability at enormous cost over time or a pure version left after great battles (the deaths of dragons are particularly likely to leave them as the body decays, explaining the particular dwarven interest on top of their shared mountain affinity). This stable mana crystal acts as a facsimile of a soul, continuing to passively generate more. It can also exist inside a soul, where it is instead known as a mana reactor and invariably associated with sapient undead, which is approximately as effective as the soul itself was in terms of mana creation, unlike the much weaker magical output of a physical object.[/quote]

like, does it actually add anything to the character if they have a random chance to harm themselves or make things that may randomly not work when tested, as opposed to e.g. taking a lot longer and having to make multiple attempts before things work as tested?

(and 'explode and destroy the item' seems like it would be limited then to 'I keep trying to make flaming swords')