We often hear people (Malaysians, particularly) use the words "that's mean" when they actually want to say "that means".
So what? It's just a little grammar mistake. Yeah, but if you ever get caught saying that in a country other than Malaysia, you might just offend the person you say it to.
When you break "that's mean" into its original form, it becomes "that is mean". So in fact, you're saying that something is cruel or offensive.
The next time you catch yourself saying "that's mean", simply correct it by saying, "Sorry. I meant, 'that means'." Unless you really do mean it.