In the sometime. There is a fresh units left in the loaded tube and the operator didn't observed it. then use this tube to put the passing tested unit. it is cause of mix unit.
For double flow testing, prepare the another color tube to put the passing tested unit of first option. And use the clear tube for passing tested unit of last option only.
Make the tube for double flow testing so that operator use to put the passing tested unit to prevent mix unit that occur from unit left in the loaded tube.
Found the fresh unit left in the loaded tube and found mix unit problem.
No found mix unit problem that cause from fresh unit left in the clear tube.
make a permanence different tube that use for passing tested unit.