The five arithmetic mistakes that account for most of the confusion in this category

Collected from the threads in this category over the last few months.

**1. Treating syringe units as a mass.** They're a volume marking. Meaningless without a concentration.

**2. mg/mcg slips.** Factor of 1000, and it reads plausibly either way.

**3. Assuming U-100.** Usually true. Silently wrong when it isn't.

**4. Transferring someone else's unit count.** Their number encoded their reconstitution, not yours.

**5. False precision.** Calculating to a resolution the syringe cannot deliver, then treating small differences as meaningful.

What unites all five: none of them produce anything that looks wrong. The solution is clear, the syringe reads normally, the arithmetic is internally consistent. There's no feedback signal.

That's the whole argument for using a tool that holds all the variables at once, and for writing the concentration on the vial rather than the mass.

Evidence tier: arithmetic and observation of this forum.