What are some odd mechanisms that make for "impossible" crime stories?

[unnildecium]

Many of the "impossible" crime stories are the Locked Room type, or wher a victim is "known to be alone at time of death".
Some mechanism has provided a delay for the culprit to be elsewhere establishing an alibi.

One had a telescope lens positioned to light a fuse only when the sun was at a certain height. Looks like victim died of dynamite accident while perp was at a party.

One, where the man in the cabin doorway was stabbed dead but there were no tracks in the snow - Solution, a freak accident, where an icicle breaks loose and stabs him, then melts in his warm body.

One, a guy is left unconscious with a small pot of water over the stove's pilot light - eventually the pot boils over, putting out the flame and the cabin fills with gas.

[Kivick]

CSI episode where a guy was shot with a bullet made of frozen meat.

[rjk]

I think it was John Dickson Carr who had an ice bullet in one of his stories.

Poirot investigated the murder of an executive who was shot in his office. The window was open, but on a sheer wall with the few other office windows too far away to escape to, and facing a windowless warehouse wall. The solution, of course, is obvious. (If you happen to be Poirot.)

[unnildecium]

That reminds me of one where a shot is heard and the assistant rushes into the lawyer's office, finds him shot but the client is calm and has no gun. The cops search him and the room and the grounds and decide the bullet, however unlikely by angle and caliber, must have come from a far building.

Actually, the client had made a simple "zip gun" out of nuts, bolts, and a small soft aluminum tube. Which he swallowed.

[RealityChuck]

Spoilers galore:

"The Tea Leaf," by Edgar Jepson & Robert Eustace in 1925. Murder in a turkish bath, but not murder weapon found.

SPOILER:

the murder weapon was a knife made of dry ice, which evaporated.

"The Sweet Shot" by E. C. Bentley -- Man killed by lighting on a golf course on a clear day.

SPOILER:

Explosives in the head of the golf club

Both from the excellent collection, 101 Years' Entertainment edited by Ellery Queen.

[Leaper]

One of my favorites is an Ellery Queen novel (it was one of the ghostwritten ones, but I don't care). A man is found shot in the head at extremely close range in his house in a suburban housing development, an apparent suicide. It's ruled as such because the only door was doubly bolted from the inside, with no chance of tampering. The only window was locked from the inside, with no chance of tampering. The fireplace had a fire going in it; besides, it's way too small for anyone to fit into it. All four walls, the roof, and the floor are solidly built and impossible to penetrate without leaving obvious traces. And since this is just your standard housing development, there's no way a secret passage could fit anywhere. The solution is one of the most ingenious I've ever read. Do NOT read this unless you never, EVER plan to read "A Room to Die In."

SPOILER:

The killer was the contractor who was working on (and owned) the housing development. He secretly sold the house to the victim lived in to him when he was in desperate need of money. Now he needed money again, and the only way he could get it was to get back the house to resell, and to do that, he needed to murder his new tenant. So what he did was murder the man in the section of house he used as his study. Then he BUILT A WALL AND A DOOR, creating a four-walled room where there used to be a three-walled room with a space opening out to the rest of the house. He simply built the door with the locks in it second to last, and then put in the last section of wall as the last job, from the outside, of course, creating a suicide room that no one could have exited.

I was floored when I read that. It was one of my best moments as a mystery reader. Wow.

[Leaper]

Oh, I almost forgot that the TV series "Monk" has a couple of doozies of the "the only person who could have done it couldn't have done it" type: the second season premiere, where a popular high school teacher seems to have committed suicide, but was actually murdered by her lover, a married fellow teacher. But at the time she plunged off a clock tower, he was proctoring the SAT's in front of twenty students.

Then there's the latest episode, where three mail bombs were sent out, all postmarked within a week of their arrival, which Monk is sure must have been sent by a man who's been in a coma for four months. He's certain there's no accomplice whatsoever involved, and the coma is genuine and deep. So how'd he do it?

And I won't even mention the skydiver who drowned in mid-fall...

[Vlad Dracul]

Larry Niven wrote a short story (might have been "The Patchwork Girl" but I won't swear to it), in which a man was shot with a laser through the window of his hotel room on the moon, but the only suspects could not possibly have been outside when the shot was fired.

SPOILER:

The killer had made a mirror out of ice, and left it on the lunar surface. He fired the laser from the window of his own hotel room, bouncing it off the mirror and into the victim's room. The heat of the laser vaporized the mirror after a couple of seconds.

[Aviary Buddy Stalk Anatomy]

Originally posted by Leaper
And I won't even mention the skydiver who drowned in mid-fall...

That one was a little less then clever -

SPOILER:

two of the many witnesses simply lied, one being out to get Monk.

However I did like a recent one where the acrobat couldn't have done it, her leg was in a cast she set herself last week. Sounded supicious, so they x-rayed.

SPOILER:

It was broken, all right, but she had faked the injury a week ago, and she really broke it after the murder.

That one was so cool!

[unnildecium]

One classic one I just rediscovered, an expensive Romney oil has been stolen. How did they get it out. Both viewers since it was last seen were searched.

Answer, it was rolled up in one of the rooms high up windowshades. It wasn't stolen to sell, but for ransom for returning it.

[Balance]

Originally posted by Vlad Dracul
Larry Niven wrote a short story (might have been "The Patchwork Girl" but I won't swear to it), in which a man was shot with a laser through the window of his hotel room on the moon, but the only suspects could not possibly have been outside when the shot was fired.

You are correct, it's "The Patchwork Girl". Niven's other notable locked-room mystery is "ARM", in which a prominent inventor is found murdered inside the field of a device that apparently slows time. The scene is in the penthouse of an enormous skyscraper, the roof entrance is under heavy surveillance (which recorded nothing), and the only entrance from below is locked from inside. The only person alive in the place is an unconscious girl with a freshly-amputated arm, which she lost to the machine. The detective is convinced that she isn't guilty...but there was no way anyone could have left the scene.

SPOILER:

There was a second machine. The killer stole it and used it to zip past the surveillance cameras between frames, then jumped off the roof with it, trailing a thin rope to provide drag. From his perspective, he simply drifted to the ground and ran off. Anyone watching would have seen a strange object plummet to the earth, then zip away at a ridiculous speed. He also lost an arm while trying to reach into the first machine's field; he had it replaced, but had to use the stolen machine to accelerate the healing process so he wouldn't have fresh, suspicious scars the next day. The detective caught on because the killer was diabetic, and went through a six-month supply of insulin overnight, leaving his implanted tank nearly empty.