So let's say the defendant, realizing its cheaper to pay a handwriting expert $3k than it is to pay his ex wife $20k, shows up in court and testifies "I never signed that document". He then puts on handwriting expert who testifies "Yep, I have 50 years of experience in authenticating signatures and that's not the Defendant's". Then our OP gets up and says, "I am just here to tell the truth. Defendant signed it in front of me, after I looked at his driver's license, and the only reason he's saying he didn't sign now is to get out of paying his ex-wife. I, on the other hand, only care about my $10".
Is the court going to simply disregard a vetted expert and the party? It might. But the judge might also think, "Boy, it sure seems like in every forged notary case I have, the notary says he checked the license and just wants his $2. Something sure seems fishy here." Result - trouble for the OP.
Point being, "just telling the truth" is great and all, but not 100% effective in a trial situation. So to complain about the "tone of the answers" pointing out a small, but extraordinarily serious outcome is possible is uncalled for. Just look at all the criminal law threads where people are told they "could go to jail" for stealing a $10 item. I don't see any of you making the same arguments there, and I don't see a lot of people in prison for shoplifting a DVD!
NB: Being a law student is arguably even worse than being a layperson in court. The false sense of knowledge instilled by school can be a very dangerous thing. Law school prepares you to "think like" a lawyer; its purpose is not to prepare you to give courtroom testimony.