As the notorious witch Hermione Weasley wrote several times throughout her career, in order to cast a good spell you must ENUNCIATE THE WORDS. Which means no stutter, sniffing, chuckling, tickling, whimpering, gagging, swallowing, whispering while trying to cast a spell. Any mispronunciation can turn a harmless charm into a hex. Anything less than a clear throat, a strong voice and perfect pronunciation won't do.

This is the second most common reason behind accidental self-maiming among witches and wizards. A sleight of hand in the wrong direction and you might not have that hand to sleight in any direction at all. Mishandling is also a common cause of explosions, missing teeth and black holes in general.

Not as common as the above, a broken wand can cause, however, a whole lot more damage than one can imagine. It is commonly accepted amidst the greatest Spell Inventors of our time that a Broken Wand Won't Do. If you have one, please... Don't. Just don't.

This could be inserted into "Mishandling of the Wand", but it was agreed in the Spellcasters Convention of 1111 that a reason of force majeure is, well, major than a mere wrong sleight of hand -- since the latter is caused by incompetence, while a very strong urge to sneeze is a force majeure that no spell undo. So in short: do not attempt to cast a spell while sneezing or something of the sorts.