He indirectly caused an airline disaster. Jessie asked a good question, "How many died because of us?" He wasn't even factoring the airline disaster to that question.

We don't know do we?

However, he seems to prefer to do things the hard way.

So far, so industrial-scale good. However, our chemists quickly run up against a DEA wall when they find that the anhydrous methylamine they need to do the reductive amination is also a controlled substance. Time to think about producing it from ammonia (NH3) and methanol (CH3OH)? No, it’d make better television to steal it.

http://weakinteractions.wordpress.com/2009/07/27/the-science-of-breaking-bad-a-no-rough-stuff-type-deal/#more-244

but then he's done things that haven't been done before in the field of chemistry.

At first glance, Walt’s employment of mercury fulminate (he later refers to it as “fulminated mercury”) seems to be a little unrealistic. He presents Tuco with huge crystals, and if he was that good a crystalliser he’d have won a Nobel Prize all by himself. Previous attempts at crystallising mercury fulminate (the ones that didn’t end in explosions) have only yielded crystals less than 1 mm in length, and it wasn’t until 2007 that crystallographers succeeded in getting a definitive structure. In addition, as I’ve already mentioned, the stuff is incredibly sensitive to shock and friction. Both Tuco’s minders and Walt handle the bag quite roughly, and even if it made it to Tuco’s desk intact the explosion that Walt causes would certainly have set it off.

My best guess, artistic license aside, is that Walt encased safe amounts of mercury fulminate in some kind of crystal to give the appearance of meth whilst retaining some explosive power.

http://weakinteractions.wordpress.com/2009/07/21/the-science-of-breaking-bad-crazy-handful-of-nothin/#more-241