Until only a few years ago a barrister could not be sued by a client for negligence in their conduct of a trial. Ultimately the policy reason behind it was that doing so would lead to satellite litigation challenging concluded decisions.
The principal authority for the proposition was Rondel v Worsley  1 AC 191 (link is to the Lords, though the Court of Appeal judgment is arguably better and the transcript certainly funnier). Mr Rondel died last month and his obit in the Guardian is fascinating.
Rondel always maintained he had not been properly defended, claiming that he had not cut his victim's ear, as charged, but merely bitten off part of it, and in 1965 attempted to sue Worsley* for negligence. Had the case been tried on its merits, Rondel would have quickly lost...
...With the help of a solicitor, the London School of Economics academic Michael Zander*, he appeared on his own behalf in the court of appeal. Zander prepared an American-style pleading of the case running to more than 100 pages, but was not allowed to read it to the court. Nor was he allowed to answer submissions made on Worsley's behalf, and so Zander prepared another statement for Rondel to read out. The court did, however, allow Rondel, a yoga devotee, time out to stand on his head in the corridor to clear his thoughts. Unsurprisingly, he lost
I think I knew he was acquitted of masterminding the Spaghetti House siege but I had no idea he was the notorious Peter Rachman's muscle and hence mixed up in the Profumo affair.
*Worsley is still in practice. Zander is known to many practitioners as the author of an academic and somewhat ponderous guide to civil procedure.