With all of the money that you’ve handled throughout your entire life, isn’t it theoretical that counterfeits may have unknowingly passed

Yes.

When I was selling stuff at flea markets over some years I did get handed a few counterfeits,  one the bank spotted when I  deposited and two of them when I noticed the color was significantly different from what should be. 

We tried to make a deposit in the ATM and there was one $100 bill that it would not accept. It looked correct to me. Color blue band, watermark, the band inside, etc. 
Wal-Mart's self-pay machine took it. This post was edited by my2cents at October 1, 2019 9:05 AM MDT

Having fondled paper money for some 60+ years, I can detect fake cash. However, I never have found any. Wait. I was given a blank penny from a Walmart cashier once. It had a rim around the outside like a normal penny, but no stamping. I still have it. 

Since I was a cashier and could care less about that nonsense, I'm sure I did.  No one sets out to launder money.  They are victims of some grifter.  We had insurance.  I'm sure Wal-Mart, etc. could handle the insurance claim.  PLEASE....  If it were a mom and pop store, I would be concerned. 

But for WAL-MART????   I'm sure it is not my job to police people's cash.  I don't work in a freaking bank. This post was edited by WM BARR . =ABSOLUTE TRASH at October 2, 2019 9:02 AM MDT

I know for a fact I have and did.  A place I worked at had the FBI and Secret Service interview us over a string of counterfeit bills come into the store.

I wouldn't be at all surprised if in the course of my lifetime of handling money that counterfeit bills have been passed.

Probably I have, though I doubt it.  I have refused a number of times to take counterfeit Bills however.  Never knowingly have I passed one though