An old headcannon of mine was that Tails just can't lie. He can't bring himself to ever say something that isn't true whether it's a white lie or a hypothetical bad faith lie. My thoughts were that dishonesty goes against his one of his character traits of being loyal, and a part of being loyal is being honest. Since I saw one of his biggest traits being loyalty.
Now while I still see one of Tails' biggest traits being loyalty, the rest of my headcannon has shifted a bit. Now the way I see it is that Tails can lie, but he's very uncomfortable doing so and has very obvious tells when you know what to look for. So it's a bit more nuanced. He doesn't do it often, but when he does he doesn't have bad intentions. He usually does it when he feels the need to 'protect' himself.
When he feels like he messed up on a mission big time, so bad that he can't even bring himself to apologize or to explain all the things that went wrong on his end. He just puts the blame on a random badnik or some sort of other interference he couldn't control. Because he fears that if he messed up too badly that everyone would leave him and he'd be all alone again, like back on West Side Island.
Or when he hasn't been taking good care of himself. He's only eaten granola bars and drank over a dozen coffees over the last several days. He's barely slept, but he musters up whatever little energy he has left to mask himself.
He's not just lying to other people sometimes, sometimes he lies to himself. Sometimes he believes in his lies more than other people do.
The only one who can always tell is Sonic. Too bad Sonic isn't always around to catch him when he falls into his own pit of lies.
