Coincidental magick only works in the undefined areas of reality; if you look out through a window to see if there is a trashcan below before doing any magick, it is no longer possible to jump out of it and coincidentally land softly in the trash (unless of course there actually was a trashcan there in the first place). Whether things (and people) who appear in coincidental explanations existed before or not is impossible to tell.
Magick cannot normally be dodged, since the mage usually just decides to hurt someone, not to create a magickal effect directed in a certain direction. The exception to this is technomagickal talismans, which usually are made only to shoot straight, in this case the normal firearm rules apply (this is also the case for magickal melee weapons etc.).
Technomancers can build gizmos or write programs to store rotes, which makes them easier or faster to invoke, in addition to making them predictable (they work the same way all the time). They are not real talismans, just extremely specific foci. In addition, this often makes vulgar magick look more static, avoiding many witnesses and sometimes even becoming coincidental.
Magick which causes something that disobeys the rules of local reality is vulgar. If no unawakened being observes it, it will be vulgar without witnesses. If a mage uses true magick disguised as stage magic, it will be vulgar without witnesses. Perception magick can be vulgar if the mage acquires information he shouldn't have, like finding out a secret password with Mind (this can be made coincidental if the mage is "lucky" and finds it scribbled somewhere) or sees a distant place using Corr 2. Although magick is coincidental in the Umbra, any magick crossing over into normal reality will accumulate paradox.
Paradox works against talismans and magickal creatures too, and generally damage them. Magickal beings gradually wither and die, and talismans loose their power unless recharged. If this is not done, talismans may break down. In the same way, the number of successes which appear when a mage creates a Pattern will affect its longevity and stability; a being created with only one success will be weak and sickly, while a being created with five successes will be both normal and vital.
Paradox is usually coincidental, or at least undetectable for sleepers. At higher levels it becomes more and more drastic, and large backlashes are so severe that they are almost vulgar. Usually Paradox covers its tracks, so that nobody can tell afterwards what happened. If something paradox-inducing happens, but the cause is in the past (and thus cannot receive paradox), Paradox will hide or change the outcome; a dragon rampaging down fifth avenue in NY would become a huge terrorist attack. Paradox can never be controlled or predicted by any means.
What is possible with a certain sphere depends to a large extent on the paradigm of the mage. Some mages may do different things from others.
The Traditions lack any central organization, although some groups within them have some measure of it (like the anarchistic network of the VAs or the hierarchies of the Rosicrucians of the OoH). The Traditions have never been unified enough to act as a whole, and individual chantries, cabals and mages follow their own lead. The main chantries of the traditions are simply the largest and currently most prestigious chantries, they have no control over the tradition. The Council of Nine is not acknowledged by the large majority of mages, many have never even heard of it.
Study points are specific for one area, such as Cosmology, Spirit or Matter, and cannot be used for anything else (some areas might overlap, depending on the kind of studies done). Mages may gain study points in certain areas during play if the ST finds it useful (if a mage reads up on science before participating in a debate, he might gain one science study point).
Dream also opens up the mind of the mage to outside (or rather, subconscious) influences. Arcane works by coincidentally removing records of the mage from file, making people overlook him or simply miss him. While the mage can partially control this, at a high level it is very hard for the mage to keep any friends and contacts. It is possible to raise backgrounds during or after an story if it fits; a mage may gather more books to enlarge his library, invest money to gain resources and if the mage has managed to befriend a dragon he will have gained a contact. However, it is worth noting that the scale is logarithmic, it is much harder for a mage to advance from 4 to 5 than from 3 to 4.
Mages of different traditions can use the libraries or tomes of each other, but in general have a hard time understanding them. They have to succeed in an intelligence + ability test, whose difficulty depends on the level of the sought information and how readable the texts are for an outsider (many hermetic scholars do their best to complicate and hide facts so that nobody else can find them, while some Adepts go out of their way to make slick multimedia presentations). Typical abilities to use would be: Meditation for the AB, Culture (Arabic) for the Ahl-I-Batin, Theology for the CC, Expression for the CoE, Intuition for the Euthanatos, Occult for the OoH, Technology or Science for the SoE, Survival for the Verbena, Computer for the VAs, Psychology for the NWO, Economics for the Syndicate, Technology for ItX, Medicine for the Progenitors and Science for the VE.
Magickal damage is aggravated if it is vulgar magick (and the effect should cause it, such as a fireball or invisible forces breaking bones, but not if a normal knife appears out of thin air and makes a slice), but if it is coincidental the type of damage depends on the coincidence (a flamethrower will make aggravated damage while a falling log or an accurately thrown knife may cause normal damage). Damage can also be made aggravated by spending Quintessence (the knife hits an artery or a ligament, the falling log crushes the skull of the victim).
See also my version of entropy.
I divide forces into physical forces and abstract forces instead of high and low forces. Physical forces are objects in their own right, such as light, fire and electricity, while abstract forces act through other objects (or other forces) and control them, such as momentum, gravity, heat and pressure (not to mention the pure elemental forces). Physical forces can be created using Forces 3, abstract using Forces 5 (the damage is unchanged, although it is possible to increase the damage of a physical force by using a higher sphere level).
Forces at level 5 also allows the creation of new kinds of forces (just as Life 5 and Matter 5 does).
Two patterns may be converted into each other by using a conjunctional effect of Matter 2/4, Life 4 and/or Forces 2/4 (Turning a human into light would be Life 4 Forces 2). This corresponds to the Infinite Loop rote in the main book.
Prime 3 allows the creation of "hanging spells", magickal effects imprinted in reality or in items, and short-lived talismans.
Personal quintessence and Tass may be used in pattern magick to replace the normal Prime 2 channelling. This will make the pattern much stronger, and usually make it permanent if it is Matter or Prime (resonance will of course be strong).
Spirit 3 also allows the mage to drag other people or objects into the umbra. Mages of some traditions are able to wander the umbra mentally, without moving there physically (the body is in a sleep- like state) or using Mind (Such shamanic journeys are far too common in folklore and shamanism to require high spheres).