This is a list of the common causes of print failures, with typical symptoms, and where possible how to fix it.

1) Cloudy resin tank: print failures won't necessarily show easily identified symptoms, but of course it's easy to see clouding in the PDMS if you empty the tank and look at it carefully. Make sure to scrape the PDMS firmly with the spatula before each print to minimise clouding, since resin pigment settles and it probably speeds the clouding process if not properly mixed in the resin.

2) Settled resin pigment: As above, failures from settled pigment won't necessarily show any distinct symptoms. It's easy to to scrape the full PDMS surface before each print to ensure there is no settled pigment, on it's own it can ruin prints, but it also seems likely it speeds up the clouding of the PDMS as well. So 

3) Dirty mirror, or resin tank: If you have holes in your parts that appear in consistently the same spot on the build platform, then it very very likely you have something on your main mirror, on the underside of your resin tank. If you have cured resin on your main mirror, that is going to be very very hard to remove without damaging the mirror, because laser mirrors are "front surface" mirrors with the fragile reflective coating on the front of the glass instead of the rear. In the case of cured resin, you'll probably just have to replace the mirror - either by returning the printer for Formlabs, or you could replace it yourself; see Replacement Components. Otherwise if it's just dust, clean if off very gently with a lens cloth.

4) Misaligned resin tank (typically after replacing PDMS)

5) Laser failure: Currently this is the Form1 bugbear, laser failures are happening a lot. The majority of laser failures seen on the forums are progressive, prints fail in more and more dramatic ways over time, until finally you get nothing adhering to the build platform, just sludge in the resin tank. Unfortunately this means it can be difficult to diagnose initially, since at the beginning when the laser first starts to degrade print failures can be quite "soft" - So if you've ruled out everything else and suspect a laser failure, then keep printing - and see if your prints get worse. For reasonable sized parts taking more than 4 hours to print, it should only take maybe 3 or 4 parts to observe a definite progression in failure modes, you may well get to the nothing on the build platform after only 1/2 dozen or so largish prints.

Once your laser failure has progressed sufficiently - it's very very easy to diagnose, by looking at the shape of the laser "spot" on a piece of paper in place of the resin tank, while running a print with the build platform removed, and the Form1 cover up. (use a magnet to fool the cut-out switch). A "final stage" failed laser spot will have a squarish profile. This is due to the defocused torch like beam from the failed laser being squared off by the two galvo mirrors. One mirrors trims the sides, and the other mirror trims the top and bottom.

At the moment the only solution for a failed laser is to return the machine to Formlabs. At this time (25/05/14) it looks as though Formlabs have changed the laser module, so it may be that future machines won't see this issue as often.

6) Galvos: When a galvo fails you will see stretched out prints in X or Y dimensions depending on which galvo failed. Prints may also "slide" off the edge of the build platform. Stretching can be static per print, or even increase during a print. Thankfully the galvos in the Form1 have been identified and sourced from third parties, so if you are out of warranty, it's actually fairly cheap (about $100US+shipping and taxes) and simple to replace them. See Replacement Components and this thread https://support.formlabs.com/entries/41140023-Swapping-out-a-bad-galvo-the-PhenixTech-PT-20K-is-a-drop-in-replacement-video-of-tuning