In U.S. court, what are the consequences for falsely pleading the fifth?

you can choose not to answer a question by pleading the fifth but if you answer other questions then yes it's illegal and you can either server time or pay a fine depending on the case

If you are asked a question and you plead the fifth the Judge tends to consider if that even makes sense. In all honesty they might even dismiss everyone if it is a jury trial and ASK about the question and how it is going to incriminate you. If you DO successfully plead the fifth when it doesn't apply my guess will be you will simply be found in contempt, over perjury.

Edit: Apparently SOME people need to read the fifth Amendment. The point ISN'T to make it so you don't have to testify, if so imagine how pointless Courts would become and especially how pointless subpoenas would be. Why waste the time issuing it if the person can just say I'm not in teh mood to tell you anything. Just send an invite and say if you feel like testifying then come on up and do it. The Amendment actually specifies SELF-INCRIMINATION and that is all it is for.

Nothing, you will simply be asked the question again.

If you still refuse to answer, then you are held in contempt of court.

But no, it is not perjury because perjury is false testimony. Pleading the fifth means you refused to testify. You can't perjure yourself by standing silent.

It's not perjury at all. "The 5th" gives you the right to not incriminate yourself. There is no and can be no penalty if you think what you say might incriminate you even if you are wrong.

If you are on a trial there is no penalty for refusing to testify, that's the point of the amendment.