Wanting an all-in-one lens.
This is a killer for me when it comes to people getting an interchangeable lens camera. The whole point of an interchangeable camera system is to, well... CHANGE your lenses. There are lots of choices available and they all give you fairly different experiences. Different focal lengths, different apertures, varying levels of build and image quality and especially price. Demanding a $1500 all-in-one lens will severely hamper what pictures you can take. You miss out on the wide and/or long end, you can't stop the aperture down as much, and in the end... it's gonna be one honking-sized lens. I don't know what else to say about this topic, but people need to be more adventurous. Don't worry about your sensor or lenses getting dirty (I don't mean be careless, but it isn't need to be treated that special). Buy a fast prime, get used to what it can do. They're small, fast, and fun to use. People think that having to walk to zoom and swap between other primes is that much of a pain in the ass... it isn't, just stop being lazy.
Sure DSLR sensors are much more capable than the smaller sensors used in point and shoots, but 50% of the people at furry cons would be better off with a point an shoot IMO. If you're not taking full advantage of the sensor because of sloppy skills, using low-quality lenses, or not taking the proper precautions when post-processing pictures and deciding what shots to keep... a higher-end camera is not for you. Spending thousands of dollars will NOT make you a better picture taker. Look at something that really fits your style of shooting.
Spending too much on gear (also, gear lust).
This kills me at conventions. People running around with tens of thousands of gear strapped around their body... it makes me nauseous. Not only are they going to take little to no [good] pictures, they're going to scare away both other photographers and people (fursuiters). You don't need much stuff to take amazing pictures--most of it boils down to purely the photographer. Using a $5000 camera body and traveling with $10,000 in lenses is NOT needed at most events, let alone a furry convention. Stop trying to show off and stop blowing your wad on gear (or renting it). If you're sufficient with using a 50mm, or 24-70, or 18-55 or whatever, that's fine... but to showoff your camera-cock to everyone is uncalled for. Go away.
Worrying too much on what to buy
Sure, hearing from other people how certain items are is just fine, and reading reviews to get a handle on how an item performs is a responsible thing too. But spending months of time squandering on what to get... you're now missing out on what just USING the lens/body would do. Just buy it! Have fun! If it doesn't work after spending quite a bit of time with it, sell or return it (or rent the item for a week). Read below for related issues.
Being obnoxious with your gear. (being "THE" photographer)
This goes along typically with the guy who has a butt-loads of money worth of gear. Stomping around with a huge camera setup (especially with huge flash brackets, flashes, and flash accessories) is ridiculous. I like being VERY discreet when taking pictures. It allows for me to capture a much more candid and innocent moment, rather then the typical "boring" as hell shots most people take of fursuits (people just standing/looking blank/zero expression). Flash (or expensive items) may be required with your style of shooting or depending on the location of the shooting, but ALWAYS needing it is a fallacy. Keep it simple.
Judging items solely on specifications and charts from reviews
Like I was saying earlier with people being indecisive about buying gear... one of the common aspects that are spent too much wasting time on is with specs and review performance. Comparing how many aperture blades a lens has or that a lens performs a *pinch* better on a MTF-resolution chart is a total waste. Things like vignetting can be easily fixed in post if you wish, but in some cases you might actually want to ADD vignetting (the same can be said for barrel/pincushion distortion)! Don't be bothered with this detail. Same goes for resolution (sharpness). Lenses from the 60's and 70's can even perform decently on modern gear. Pixel peeping is a gigantic waste of time and NO ONE will notice if a picture is properly sized down via post or the web. The only thing that has really been improved upon over the years was the addition of [silent/fast] auto-focus and resistance to flaring and the softness and low contrast that can be imposed by it.
Buying a lens and then shooting only focus charts and worrying about AF fine-tune and returning several copies to get the "perfect" lens is also a huge waste. Go out and SHOOT with the damn thing, it'll be fine. Very rarely do completely-fucked up lenses make it off the production line. Varying tolerances are permissible by manufacturers and your fucked-up "lens testing suite" isn't going to give you ANY good results. Like I said, just shoot what you normally would, or would want to with your new lens purchase. C'mon guys.
Thinking you need the best gear
This is far from the truth. Many budding and beginning photographers begin dreaming about expensive high-end gear and become disheartened when they have to use "lesser" things. Jared Polin, a youtuber has a pretty good channel that helps beginners with gear choices and how to process pictures. Low-end DSLRs these days are packed with extraordinary features now... don't worry. Fantastical lenses can be had for cheap too. With that in mind, and with shooting RAW and properly handling your photos, you can get awesome pictures. I would gladly spend my time with someone who actually knows photographical skills and has a $300 body, rather than someone who THINKS they know everything and has a $2000 body.
The best camera is the one that you have with you. This is a point that is driven home really well with another youtuber named Chase Jarvis. Check out his channel, he is very charismatic and is a fun guy. He deals with VERY expensive gear at times, but is very friendly and can appropriately help just about anyone. Having a plan on what you want and what you need is also crucial. Starting out with a cheap zoom and figuring out what you want to shoot from there is the best way to go about things.
I know there are other things I could probably bitch about, but I wanted to get this off my chest because it is very annoying.
Have fun shooting!