This falls under the heading of Reading Is A Good Thing with a subheading of Read Carefully 2 Or Even 3 Times Before Starting.
I'm about 2/3 done with my current project, a Victorian house. I've been working on the "trees" on the sides and am nearly done. I just noticed last week that 2 symbols use the same blended threads and thought, Huh. Didn't think too much about it because one was a half cross and the other a full cross, so okay.
Then last night, as I was getting even closer to the end, I found out that the half cross was supposed to be *3* strands, not 2. GRRRRR!
So, I spent the time I should have been nearly completing *all* the trees going over the half crosses a 2nd time. Thankfully it wasn't on the full crosses, because that would have been much harder and I probably wouldn't have bothered.
And yes, I can tell a difference in the areas I have restitched, so I'm glad in the end that I put in the effort.