Spells so hung are usually of a guardian nature, but may do great harm. Explosive runes, sepia snake sigil, fire trap and symbol spells are the most common, and rumours abound of stranger and more deadly spells that have been devised to guard a mage's rune.
No two mage runes are identical. A design chosen for a rune simply won't function as such if it accidentally resembles on used by another mage (either active or in the past). Most who cast spells in Faerûn choose one rune for their entire lives, never changing.
By the will of divine Mystra herself, the High One visits a heavy curse upon those who falsely use the rune of another creature. as the old apprentice rhyme reminds us:
Whenever magic one doth weave
'Tis never, ever, wise to deceive.
'Tis never, ever, wise to deceive.
While there are more mechanical details available in the Forgotten Realms Campaign Setting book, keep in mind that we DMs can and do play around with this. If someone gets inspired to create a rune for their character and post it on the forums, rest assured that the Curse WILL come and get you if you "use it falsely", no matter what your Will saves might have to say about it.