I thought I would list out a few scenarios in my professional career that made things really challenging even on days I thought I were at the top of my game.
(1) Mixed Lighting Scenarios
Of all scenarios, I thought I would mention this one first. It's probably one of the few that is avoidable and with experience, will get easier but never easy. Some of these places include concerts and lecture halls. The reason why these places can be challenging is due to the variety of lights with different light temperature that will cause your photos to look very different from one position to the next. In concerts, the lighting and lighting temperature is constantly changing for effect. As for lecture halls, I find that many lecture halls has weird combination of hot spot lights mixed with cool flourescent lights. These mixtures of temperature not only cause shifts in lighting temperature but also can add a weird tint to your photos.
(2) They Just Don't Know What They Want
Ever worked with with a client/marketing or PR rep that simply not know what they are asking for. I have been in situations where the PR representatives were clear about what they wanted but just that what they described was very different from what they had in mind. These are one of the worse case scenarios and eventually you will learn to find ways to ensure you don't spend hours of hard work delivering photos that are not what your client was really asking for. As a professional photographer you can't blame the client for being unclear. You just have to work with them to make sure you clearly get what they want and that what they say is exactly what they are really asking for. It's hard, and if not it would not be on my list. I have been in situations where the client doesn't mention anything about issues with the photos until after the photoshoot or even once the post work has been applied (ex. they tell you that they wanted something in the original shot). You need to constantly get confirmation with marketing and make sure they are happy. It may seem annoying to do so but it's much better than getting a reply in email afterwards saying, "this is not what we were asking for."
(3) The Precisice Moment
Ever shot an event where you were asked to shoot something or someone at a precisive moment? While this may not be the most difficult scenario but it's definitely one that takes a lot of planning. Often, to secure the shot it will be shot with extra cameras and often with extra photographers to guarantee the shot. You will not be given a second chance to retake a shot, so must get the settings correct. Some things you can determine beforehand like colour temperature, aperture settings, shutter speeds and lighting setups, but there are situations where all the planning ahead of time can change at the last minute. Shooting celebrities or very important people in general， don't be surprised if last minute changes are really last minute. Also if it's a big event, you won't be the only photographer there. Not all photographers are courteous and play fair. Especially if you are dealing with paparazzi photographers. Not saying you will have to play dirty but you'll just have to find a way to fight your way through at times and get your shot. Remember at the end of the day it's the photographer with the best shots that count and not how you present yourself and get along with others. The bad thing about being a event photographer and not a paparazzi photography is that you will be required to juggle between both because you are not just selling your photos but your services as a whole.