Is it possible for someone to have a plan and not have one at the same time?

[Der/die AtheistIn]

Imagine that someone you know very well has an achievable plan to do something. 
The person has at least average intelligence. Not only does this person say out loud that they're sure their goals and way of obtaining said goals are good, but there is evidence to support that they truly believe that. You know them personally and you can tell.
They don't feel any pressure not to follow their plan or if they do it won't stop them. If anything, they feel more pressure to follow their plan.
There is also plenty of time.
Inspite all of this they don't succed and it's their fault. 
Let's say that they did an obvious mistake, that could've been easily avoided. They didn't fuck up because of pressure or curiosity. In fact, they should've been pressured not to do that mistake. It is something that does not go against any of their beliefs. 
It's like their trying to make a puzzle and manage to almost finish in a (relatively) short time and are very determined to finish it, but they forget to put the last piece. They're not tired, said piece is right in front of their eyes as well as it's place. The puzzle isn't complicated, it doesn't have a lot of pieces. They're also not bored of the puzzle and they're not lazy, at least when it comes to this puzzle. Inspite of all of that, they still don't see that it has a missing piece.
What do you think can be the reason behind it?