What can't you say in Parliament to anyone? May also apply to other Countries ?
Good clue: There was a famous quote by Sir Robert Armstrong to a British Cabinet meeting (discussing "Spycatcher" in 1986) which was a posh way of circumnavigating around this.

Is it calling someone a liar? The way around it is to say "I believe the gentleman is mistaken".

True,you can't call somebody(In Parliament) a Liar to their face,True again about the phrase used,
Sir Robert Armstrong was the person who quoted the Phrase: "Economical with the Truth" which brought back into common use after being rarely used since the 18th century:
Definition/History here
And the story of "That Quote" and similar stories here

The phrase " economical with the truth" is not a euphemism for lying. It means just not giving the WHOLE truth, which is somewhat different. That's why the user got away with his statements. However, Sir Winston Churchill once used the term " a terminological inexactitude" as a euphemism for a lie. This was accepted by the Speaker at the time.

'Unparliamentary language' includes more than just calling another member a liar. The Speaker will ask a member to withdraw any statement that a fellow member is drunk or misrepresenting another member's words ,too. He will not permit other language which is no more than vulgar or obscene abuse, of course, or general or irrelevant accusations of crime or dishonour it seems. Precedent shows the rule applied against the use of hooligan,rat,swine,stool-pigeon, blackguard,coward,guttersnipe, traitor and git. (Source: www.explore.parliament.uk). The rule has been adopted in other countries where the UK Parliament is the historic model for their assembly e.g. Canada and Australia