Is there a penalty in Uno for incorrectly accusing a player of failing to call "uno"?
I assume that yelling out "one card" is the way your group has decided to implement this rule:


  If [the player with one card is] caught not saying “Uno” by another player before any card has been played by other players, the player must draw two new cards.


That rule does not establish a penalty for incorrectly "catching" another player not saying Uno. You could add a house rule to this effect, especially if people abuse the rule by yelling "one card" incorrectly very often. However the house rule would itself be open to abuse since it would encourage people to mask how many cards they have, so I would only add it if people are being really annoying with the incorrect calls (and in that case why are you playing with really annoying people anyway?)