As tasks and time, which costs money, is a main focus for planning, nearly everyone you will talk with in gathering information will short change the overall effort.
This is not on purpose but as we all tend to do, the one who knows the process best may fail to mention little items or easy tasks due to their familiarity.
When you give directions or a recipe or instructions how to play a game, you might miss a street, an ingredient, a step however seemingly insignificant, ends up making a big difference if left out.
No doubt why interrogation requires you to repeat your story over and over.
The same with estimating a process for project planning. You will want to ask several people about the same process then circle back to each one who left out any steps you learned from your interview. This helps reduce the “surprises” that are an enemy to time estimates.
Remember not to pit one worker against another but simply say “I was thinking there might be situations were product isn’t shipped immediately after picking, can you elaborate on scenarios that might cause a delay?”.
Damaged box or label issues, you will be able to fill out your list of risks more fully with likely items of note and a better idea of problems that could affect your schedule.