The most common mistake made by inexperienced filmmkaers is either lack of proper planning, or abandoning those plans 1/2 way through shoot day #1. This mistake is soo common, it's actually anticipated & joked about by seasoned Film Crews & Producers.
Countless first, second, third & fifth time productions fail completely because of this exact circumstance. They don't finish their shooting day, then they run out of time, budget & resources, inevitably not being able to assemble a complete story line or film. A really bad first timer will go on to blame the cast, crew & Producers around them for 'lacking support' or 'not seeing their vision' & completely fail to realize they should have planned appropriately before hand, & then shot exactly & only what they planned for.
Usually it unfolds like this, a new Director reads an article or book about how a famous 'Master' filmmaker, 'had a vision' and forced his crew to 'get the shot' for hours, therefore proving their genius. Most newb filmmakers think thats all directing is, & thats exactly what they want to do, the first chance they get. From there, they get stuck shooting one take, 87 times for 5 hours, completely blowing their whole day. If its not the 'stuck on a shot' scenario, it will be the 'I have a great idea' scenario, where they decide (not during Prep or the Tech Scout, but in the middle of shooting) that they want to do something completely new, that no one is prepared for. They move a dolly, then the lighting set up & a wardrobe change. Before you realize it, its 3 hours later, the sun is setting, & you never moved on to the rest of your day light scenes. You didn't 'make your day' & now your movie is over budget & out of schedule.
The truth of that 'Master Director' story is, its mostly exaggerated Hollywood myth & PR, only possible for the most experienced & accomplished filmmakers in the business. In actuality, most Producers & Studios wont tolerate it, & if a Director isn't capable of providing consistent 'deliverables' daily, they will be privately reprimanded at first, not invited back for more work, second, or third, get replaced entirely during the initial shooting.
Now, you may have heard that story a lot. 20 or 50 times you've heard about that 'Master /Auteur' who demanded perfection & got it. I'm not saying its entirely untrue, but in most cases they (A) planned for it weeks or months ahead of time, & (B) had to beg Producers & Financeres for the opportunity to do it, with polished scripts & detailed story boards, far in advance. It wasn't something that occurred to them 1/2 way through shooting a scene, & it wasn't something that they executed without a lot of preparation.
So, lets say you've heard that story maybe as many as 50 times, about the worlds top, most influential film Directors. Great, 50 Directors seems like a lot, but you've got to consider theres approximately 16,000 registered DGA filmmakers, & thats just the top, union guys working now. Theres easily 1000's more non union, independent filmmakers trying to make their mark. You've probably seen a lot of these peoples work. Big commercials & episodes of shows you watch regularly, but you don't know any of their names, or the body of their film work. You may not of heard about how they are super demanding auteurs, but their work is obviously competent & quality when you look at it. These people don't achieve their success by abandoning plans & changing their minds while on set.
The surest way for new filmmakers to mess up their own projects & careers, is to abandon the script &/or the days shooting plans, & try to act like a creative genius in the middle of a shoot. It sounds cool in a story, but when its happening on a film set its usually a precursor to major disaster. Most of the time, especially with inexperienced filmmakers, it results in not enough footage shot, & only 1/2 of a finished movie.
As a new Filmmaker, either a Director or Producer, its much better to have a 'pretty good' film thats 100% complete, on time & on budget, than to have a 'Great Scene / Shots' that don't cut into a complete movie, & you're out of time, money & resources. Completion is key in this industry. To become an accomplished filmmaker, you actually need to 'accomplish' finished films, not stockpile cool footage. In most cases, quick, intuitive & inventive 'compromise' & being able to shoot & assemble a complete, cohesive scene or film, will be a far greater skill & asset to a new filmmaker, than demanding your Actors or DP 'get it perfect'. To be honest, the audience probably wont know the difference between 2 of the same takes that were just 'ok or good', but the audience will notice, if you have major plot holes, critical segments that are missing, or no final act & conclusion to your film.
So, when you are planning production & scheduling your days, you need to 'tell the story' & 'make your days'. You need to shoot enough coverage to be able to edit 'something' (hopefully what you envisioned) together when you leave that location, and you need to get all the footage you planned for, while you are there. You don't want to pay for additional re-shoots, & even if you wanted to, a lot of times the cast, crew or location just wont be available again. Missing this opportunity, & thinking you can catch up later is by far, the most major & common mistake made by new filmmakers. More often than not, this mistake results in the overall failure of a film, because the production team can never catch up with the lost time & resources, & they didn't get enough footage to cut a complete project.
Be mindful when planning & scheduling a new project. Make sure you have enough time & the assets you need at the location. Prep well & scout well, planning all your movements & each thing you want to accomplish, allotting enough scheduled time for all of it. Then when you get there, on the production day, do exactly what you planned on, & don't get distracted by changes or new ideas. Get what you came for, in the time frame you intended & try not to do anything else.