The member might mistakenly place a witness closer to the incident than they actually were. A steward must constantly go over the story, checking every aspect of it. Challenge the story as you would expect management to challenge it. Explain to the member that you are not doing this to undermine him or her, but to help put forward the strongest case. Try to find other credible witnesses who support the grievant's story.
Different witnesses see the same event differently.
Don't be surprised at this. Talk to the supervisor or company witnesses beforehand. Check out their story and write it down. You might need to refer back to these conversations at the disciplinary hearing or grievance appeal. If you can show that the supervisor said one thing to you at your meeting and then he or she contradicts this story at a hearing, you begin to build a case that questions the supervisor's credibility.