Somehow, the acid-laced cigarette that kills Nicole doesn't start melting until she smokes it, and doesn't weigh enough to make her suspect it's been tampered with.
She does actually light the cigarette and inhale the acid. I'm sure oxygen plus heat can actually instigate some sort of chemical reaction...maybe a powder which wouldn't be as heavy.
Acids that are benign until activated to heat are quite common, It is what solder flux is http://en.wikipedia.org/wiki/Flux_(metallurgy), that said. there are pretty much no plausable chemicals that will have the effect shown in the movie.
The killer actually does make mistakes thanks to Gabe. Hypothetically, Vince should have died due to his wheelchair. If Gabe hadn't have been downstairs with him, he would have had no place to go, eventually he would have either fallen into the water or been electrocuted in his wheelchair. I guess he didn't count on Vince trusting someone else with his gun. However considering that Vince wouldn't have been there in the first place without Gabe this qualifies as a either an elegant Xanatos Speed Chess or a very bad Gambit Roulette.
As somebody noted elsewhere, in the span of a few hours between the moment when they were drugged by the coffee until they woke up, the killer somehow found the time to:
Saw off Rafe's head in a clean cut;
Drained all the blood from his body;
Used the blood to write a few thousand specific numbers on a wall;
Tortured Val Kilmer and hung him from puppet strings in some warehouse on the other side of the island;
Rigged the lights to malfunction and the water pipes to burst;
Rigged three enormous harpoon bolts to fire out of a wall when a specific handle was turned a specific number of revolutions;
Wrote some letters on everybody's coats in fluorescent powder before repositioning them exactly how they were laying when they were knocked unconscious by the coffee;
Opened up every cigarette vending machine on the island and inserted into each a pack of smokes specially coated beforehand in an acid that does not harm paper but eats people alive;
Somehow set every said vending machines to dispense the deadly smokes at a precise time of his choosing;
Planted a bomb on a boat, complete with a laser tripwire on the pier leading up to it.
Admittedly, some of this could have been carried out the night before while everyone else was asleep (Although this raises its own Fridge Logic issue — then why wouldn't the killer be tired after working through the entire night?), but it's still an enormous amount of work.