In one of my stints of jury service, the accused had heavily exercised his right to silence over a lengthy period. At the subsequent trial the judge made a point of instructing the jury we must infer nothing at all from his silence and judge solely on the facts of the case, as presented in court.

I do not know what edge cases the addition of "But, it may harm your defence if you do not mention when questioned something which you later rely on in court." was meant to protect against, and how, but it is absolutely nowhere near obstruction of justice or contempt of court.