Teleportation has always seemed too easy for me.  I’ve always thought that it was sort of a easy out and cheesy way to get around withour actually having to travel (and run into those wonderful random encounters). So why not just make it a little riskier.  Hehe.  I fully admit that much of inspiration for this came from Lamentations of the Flame Princess. You’ve got wizards ripping holes int reality just summon an insane monstrosity from an equally insane dimension so why not this concept one step further.
I wanted to keep this as rules neutral as possible. GM’s should use this as means to make the party’s life “interesting”. Wizards should know that very bad things could happen when teleporting and GM’s just shouldn’t throw this in as a surprise.
When a wizard casts a teleporation spell, he’s just creating a short cut through a forgotten dimension.  Each character (including the caster) must make a save versus Magic (Tweak to the rules you happen to be using).  If they fail then roll on the following chart:

1-The character is replaced with something else.  It may look like him and even act a little like him but it’s something evil and will abandon, kill or betray the party at it’s earliest convenience. The original character may or may not be able to rescued. If the character is some how rescued then roll 1d10+10 on this chart for any additional effects.

2-There are now two of the character.  One is the character and the other is evil as above. (Note: For a really good way to handle this see the adventure A Stranger Storm in Lamentations of the Flame Princess). The evil knows everything that good one knows.  It’s up to the party to figure out a way to deal with it.

3-There are now two of the character.  One is all the good and sensitive parts of the character’s personality, the other the dark and evil parts.  For the character ever to whole again, the party needs to figure out a way to put the halves back together. (Ref: The Enemy Within from the original Star Trek)

4-Something extra comes through with party. A hideous monster slips through with the rest of the party and attacks them immediately.

5-The character’s arrival is delayed. (Roll a d6. 1: 1d6 rounds, 2: 2d6 rounds, 3:1d6 minutes, 4: 1d6 hours, 5: 1d6 days, 6: 1d4 weeks).

6-The character gets displaced from the rest of the party in a random direction. (Roll a d6. 1: 1d6x5 feet, 2-3: 1d20 x 5 feet,  4: 1d6 x 100 feet, 5: 1d6 x 1,000 feet, 6: 1d4 miles)

7-The character has been infected with a horrible extra-dimensional parasite. The character will be fine for a few days and then start showing ill effects (strange cravings, weight loss, eating too much, always hungry). Then the character will be sick and so weak that he is bed ridden.  If the situation is not solved in another week then a ravenous creature bursts out of the characters chest and goes on a killing rampage. (Yeah, you know where I got the idea for this one.)

8-The character didn’t quite make the transition.  He and his gear is insubstantial for 1d4 hours. He is not invisible but just a ghostly apparition.

9-The character’s passing has damaged the veil between realities and left a portal open.  Many unpleasant things come pouring through.

10-The character really missed the mark. He materializes 1d6 x 10 feet above the ground and then takes falling damage.

11-The character looked into the abyss between the worlds. He permanently loses 1 point of Wisdom and gains a random insanity. There’s a good chance that the character’s hair turns white from the horror’s that he has experienced.

12-The character’s mind has been jarred by the trip. For the next 3d6 rounds, the character acts as if he is under the influence of a Confusion spell.

13-Something lashed out at the character during the trip.  The character takes 3d6 damage.  This wound leaves an odd scar that will never go away.

14-  The character loses a random item.

15- A random item the character is carrying becomes enchanted. Considering where this happened the enchantment probably won’t be a very pleasant one.

16-The character is plagued by nightmares of what he thinks he might have saw. For the next 1d8 days the character cannot get a good night’s sleep.  He is never considered rested and won’t regain spells.

17-The character is possessed by an entity. It generally hates the character and wants to go home but that doesn’t mean it won’t torment the character or cause some death and destruction while it is in this dimension.  The character may have strange dreams or wake up in strange places covered in blood.  Getting drunk til the character passes out won’t help.  If properly motivated, the entity may even help the character if the price is right and the character is willing to pay it.

18-The character suffers from hallucinations for the 1d10 days. He will hear voices or even his companions say things.  He will see things move in the shadows.  Of course, sometimes there might really be something in the shadows.

19-The character is struck blind for 1d8 days or until cured.

20-Arcane Knowledge.  Somehow the character gained the ability to cast a random 1st level wizard’s spell once a day.