The history of English Parliament is a great example of an existing organisation taking on functions it is not actually designed for. Originally, Parliament served two purposes- to acclaim the King (queen) at their coronation and as a sort of finance committee. Because individual Lords dispensed justice on their own lands there has always been a judicial element to the House of Lords. The King, and later Commons, used various non-aristocratic justiciars to make their own power effective and, as a by-product, to eliminate the more grevious injustices in the feudal system. Even then, however, the best Judges were often to be found in Lords. RW Southern gives a good account of the career of a Circuit Judge at various times in the Middle Ages and later and the one thing that sticks out is how little time they actually had for doing their job (which often included non-judicial matters) much less improving their knowledge (days in the saddle, stuck somewhere in bad weather, etc).
When judges were rewarded for service by elevation to the peerage it was not uncommon for King or Commons to call them back to the bench in important cases, and then the chief judges of the land were automatically appointed to Lords as Life Peers. Technically the Law Lords are not supposed to vote on legal matters but were able to speak on them. How it actually operated I am not sure; while it still leaves a lot to vote on there must be occasions when the lines blur. Technically speaking the Law Lords did not hear cases as Lords but as Judges; thus their courts were not, by the same technicality, part of Lords.

I should also clarify one thing- a parliamentarian has to be found to have knowingly lied to Parliament. We had an incident here a few years back where a parliamentarian, later to be Premier, made a series of accusations against a member of the government involving an alleged affair. The woman involved committed suicide and in the uproar that followed it quickly became clear that there was no foundation to the accusation. The MHR who made the allegation was found not to have breached the rules because he had received the information from a source that he could not have 'reasonably' expected to be biased, though the source was a close friend of the husband of the woman and they were in the process of divorce. Of course everbody was sure he knew it was biased information but without proof nothing more could be done.