OK, not actually deadly, but these deserve a mild flogging at least.
is excessive belief in one's own abilities, that interferes with the individual's recognition that those who have gone before might actually have something useful to offer. Pride manifests itself in the launcher not bothering to read the UKHAS wiki, research previous flights, or join the #highaltitude IRC channel for advice. The typical result is that the flight doesn't appear on the map (no flight document), and apparently reaches a ceiling at 12km (GPS not in flight mode).
is the apparent need to launch the balloon next weekend, regardless of preparations or the lack thereof, regardless of the likely weather conditions at the launch site, and regardless of where the payload is likely to land. Sadly, impatience is frequently combined with …
which is the avoidance of work, and that mainly results in a lack of testing. The flight itself is not a test; it should be the end point of a lot of ground testing to reduce the chance of failure as close to zero as you can reasonably get.
that the flight will be a success. Here, the launcher assumes that other people will do the tracking for him (even though the flight was probably not announced) and will go out and collect it for them (because the launcher didn't take a radio, or antenna, or at least ones that they know will work at a distance). A common factor here is having a GSM tracker as a backup, which leads to confidence that the payload will be found even if no attempt is made to track by radio.
Forgetting to take everything you need for the launch and for the chase. Forgetting that there exists a bunch of other HABbers just itching to help if only you could remember to help them by getting your tracker uploading to the internet at the launch site, and announcing the actual launch time and frequency on the IRC channel.
Ignoring the advice of other HABbers is probably the worst sin; they've already made the mistake that you're about to make if you choose not to listen. If you're told not to encase that GoPro, not to add heating pads, or (especially) not to launch yet as you're clearly not ready, then please don't assume that you know better.
The main one is “neglecting to check that your tracker is actually working before launch”. This seems to be an increasing problem, and is so easily avoided by setting up tracking at the launch site, including uploading to the internet via 3G. If you launch without working tracker you might as well not bother.
This and many other similar errors can be avoided by having a good check-list and making sure it is followed.