As we will now start to venture into more spell work and other ritual stuff, I wanted to take the opportunity to remind you all that everything we do in life has consequence.
I once ventured into a magic system where magic not only literally used your life force, but it was impossible to draw power from anywhere else. This means that anyone could use magic; anyone with no care for their own life could go out in a blast of insane power.
Mages are simply people who have learned tricks to minimize the amount of power they use and have a greater level of control over what they do, so they can stay alive longer.
Most magic is non-corruptive on a personal scale, although some can be, depending on the source and nature. However, excessive use of magic in the same area will start to weaken the fabric of reality. First, this will cause minor aberrations to appear. Soon though, magic itself will become unstable, mutations will occur, and all manner of dangerous aberrations will start appearing.
Consequences are the results of the magic you crafted. That said, magic is cast from essence (life force) so technically, overextending yourself on a spell can result in exhaustion, illness, or even death - but only both powerful and reckless/desperate wizards are likely to suffer severe impacts this way.
Those indirect consequences can be a kicker though! First off, magic is forged from Intention. Unclear or inconsistent intention can result not just in weak magic, but unreliable and dangerous results. Bear in mind that all magic has a lifespan beyond the duration of the spell you cast. For a stereotypical example, if you cast a baneful spell, the imprinted essence that makes up the 'baneful spell' still exists in the world after the spell is cast - it might 'echo' for quite a while after, or might even form into a small, non-sentient spirit - in this case, an almost certainly destructive spirit.
Magic can also attract and/or alter existing spirits, for good or ill. Spirits which 'feed' on certain types of magic are often altered by the nature of that magic - spirits shaped by destructive, violent, or malicious Intention often become hostile themselves, and the offerings/sacrifices humans like to make to these spirits, in particular, can warp a spirit's Nature, making them more human-like and more predatory. The Ura people call these 'Ulem' - the poisoned ones.
Furthermore, there is a rather-significant physical and mental impact on becoming a sorcerer. Sorcery is a matter of fusing a mundane being with a spirit into a single entity capable of powerful magic - however, this can be a traumatic process.
By convention, sorcery warps the mind and body and could result in deformity and/or mental instability. Sorcerers thus marked are then rather at the whims of their society - some are exalted, while others are abhorred. Magickal thinking is a psychological term for making a naive assumption of cause and effect without consideration of intervening mechanisms. In plain English, it is the assumption that if I do this, then that will happen even though I have no idea how or why.
For many outsiders, most Magick seems to be built on this basis, some guy mutters some weird words and waves his hands about and expects to get a lot of money or to make it rain or be rid of an enemy. Then when they fail, they shake their heads, call the Magick-users names and go on with their lives.
Within real Magick-use, this sort of sloppy thinking can lead to anything from disappointing 'fizzles' to disastrous misfires of spells. Our cultural heritages are filled with "monkey's paw" type stories of the results of ill-thought-out Magick use. While dilettante New Agers are more likely to blindly 'cookbook' a spell or ritual, some of us have been known to skip a few steps in the process as well.
Just think of the consequences of invoking some demon in a ritual designed to "keep them in their place." While I am not saying that you need to understand the physics/chemistry/etc. of each step down to the subatomic level, I am suggesting that you think through each step and each mechanism (and likely consequence) of any major working before you perform it. A black box understanding (detailed knowledge of the inputs and outputs of a mechanism and the relationship between them without an understanding of the internal details of the mechanism) is usually enough.
For instance, if invoking or evoking a deity, make sure you know the strengths, weaknesses, character, and personality of that deity. If using herbs (ingested or in balms or incense) be sure you know the pharmacological and combinational effects of each. Most of all, when going for a long-term effect think of the ecology of that effect: where it can come from and what it may cause later. You can't always anticipate all side effects, and you certainly can't always avoid them, but with a bit of work, you can give yourself a shot at handling them.
Do reality checks before you start working with magick. If you just pay attention to the beginning (the ritual or working) and the end (the desired effect) and leave the rest to wishful thinking, you are asking for big trouble.