Property of Wrapport Library, Wrapport College, Wrapport, Mareten, Mor-Thir.
Wishes and miracles are strange, powerful spells, capable of manipulating reality itself.
Miracles, of course, are infrequent blessings handed out to faithful followers, and are often accompanied by divine quests, as difficult as they are mandatory. The more powerful the miracle, the more stringent the quest. The most powerful miracles, granting immortality, powerful enchanted weapons, and so on, are simply refused, or even punished, at the whim of the miracle-granting deity. Wishes, on the other hand, seem to have no oversight; at face value, it seems even the most exploitative wish could be made reality.
That is only true in part. You see, wishes change the very fabric of reality; whilst a small or well-thought-out wish only brushes the threads of magic binding the universe, a daring wish of great magnitude tears through those threads, like a heavy stone through thin fabric. A wish for a +10 sword may indeed grant that very object, but it may also turn the earth upon which you stood into a roaring channel of destructive magic, instantly destroying you and anything else for miles around. Deities control their wish-granting for just such a purpose; their innate connection to their divine magic allows them to be very precise about their wish-granting. Wishes that take immense power are refused, or couched in such terms that the recipient must repair the threads of reality in trade.
Wishes, with no such oversight, put the burden entirely on the caster. Their intent determines the damage. It may be that a powerful magic item, capable of casting spells as a 14th level mage, can be created, but the magical power of the original caster is entirely and permanently removed. Or, a sword +6 is created, but its intended recipient suddenly finds their energy drained, and must attack at a -6, effectively cancelling out the magical sword. Of course, it may be that the wish is poorly worded, which is why there are such things as an enchanted crassbow and a magical bow staff.
With that in mind, is there such thing as a safe wish? Perhaps not, but the following steps will reduce any danger:
First, be precise. Imprecise wording or intent will end up with a useless item; a misspelled or misspoken word will be interpreted in the worst possible way.
Second, be as simple as possible. Basic enchanted items, like a +3 sword or crossbow are likely to succeed without trouble, but complex requests like sentient items are much more likely to be garbled.
Third, be logical. The threads of the universe follow a pattern; if your wish is free of loopholes, it is much less likely to affect you badly.
Fourth, and most importantly, do not wish for immaterial things. Knowledge, power, and safety are great, but difficult to attain in an instant, and a leading cause of wish-insanity. This also includes wishing for no ill effects from your wishes; all wishes have ill effects, it's just difficult to see them. Your +3 sword may have been plucked from the hands of an ancient hero, leaving them to die horribly, for instance2.
If your wish is precise, simple, logical, and material, there is a good chance that you will escape unharmed... but even then, it is not guaranteed. Wishes are fickle things, and as they say, the best-laid schemes of mice and men gang aft agley3.
 Mage, 11th Degree, Dean of Wrapport, Honerary Recipient of the Tendril of Oh Goodness, Authors of A Dissertation on Self-Cloning and Time-Travel, aka Lucious Ezmereldac Whitherfordshirebeck, aka Lightning Lucious.
 I believe this is what happened to my favorite walking stick.
 It's from a poem.
N.B. This manuscript was discovered in a sealed potion bottle, adrift in the Eastern Ocean. It appears to be written in squid ink, on tanned whale-skin. On the reverse is a fully illustrated and magically animated shopping list, including "milk [cow]", "eggs [chicken]", "those little meat things in mustard", "CHOCOLATE", and "definitely, positively NOT FISH."