How much time do you have? Seriously, I've probably screwed up turkey hunts in every way possible. I might even have invented some new methods. Most experienced turkey hunters can say that.
I could go down the usual litany of gaffes — moving, not scouting, calling too much, calling too loud — but you asked for one specific error. I'll have to go with this: Not preparing as much as possible for a hunt. That might entail not knowing the lay of the land, failing to keep tabs on bird behavior, not having the right gear for the situation, not practicing to become the best caller I can be, not recognizing a situation and acting accordingly and a dozen other scenarios.
The point is simple: Turkeys are unpredictable, and any season is a roller-coaster. You have to take care of the things you can control, which comes down to preparation and decisions. Learn your hunting property. Watch how birds use it. Practice calling so it's second nature and as realistic as you can make it. Review logbooks or think back to previous hunts, and recall how birds reacted in various situations. Buy equipment that will let you hunt hard, comfortably and efficiently.
Also, remember these words, courtesy of Jim Spencer: The difference between inexperienced turkey hunters and experienced hunters is that inexperienced guys worry that they'll make mistakes. Experienced turkey hunters know they'll make mistakes.
If you blow a hunt, shake it off, learn from it and don't repeat the mistake. The next morning, that longbeard might run to you anyway.