Make 2

What's wrong with this picture? This is an example of "make 2" and the shocking realization and re-counting where it could have gone wrong... And which one is correct? Answer: The larger one. It's the body of Robert, the bear.

Two pieces for body of bear: Seams down middle front and back.  Darts at top for shoulder shaping.  Darts at bottom for bottom shaping.

I sat stunned as I put them together to admire...  I knitted the one on the right first, then up to the top points on the one on the left.  Really, I had compared them earlier, but... it wasn't until late in the game that I saw the difference.  I counted each one, several times, and found that I had skipped a section and mis-counted on the first one.  No worries.  Except:  (1) This is a trial for a class I would like to teach in March.  How frustrating for knitters who don't knit all the time!  I guess it tells me that I'll try to keep everyone on the same page, doing the increases and decreases at the same time, etc. to avoid having to count and re-count. (2) What if there had been only "one" of these?  How many other projects are crazy WRONG? LOL
How to avoid these mistakes?  Suggestions are welcome. Maybe lots of markers are necessary to keep track of progress.  This is a learning experience for me!