quick rules question

[gsusguy]

I play with some friends that said:
If you call someone's bet on the river and the showdown happens. If the person who was calling shows their cards and they've won then the person who called them can try to muck their cards but if someone at the table wants to see them, they have to show them... Is this true? I hadn't heard it before

[emonrad87]

This is true in casinos, but it's only there to protect against collusion, and is only to be used to check for collusion.