"Incurable" or "Plain" Error

We've been talking about preservation of error: the doctrine that an appellate court will generally not overrule a trial court's ruling if the complaining party did not let the trial judge know at the time-- usually by objecting-- that the party thinks that the ruling was wrong. We've talked about one exception to this rule: that objecting that a trial court or a lower appellate court did not have jurisdiction over a matter may be objected to at any time. An objection to the present lack of jurisdiction over subject matter in the appeals court that one is before at the time does not need to be made to preserve error.
Appeals courts have, from time-to-time in the past, recognized "incurable" error (which is what Texas criminal practice calls it) or plain error (what the feds call it). These are errors that are so bad that they don't need to be preserved. They are plainly wrong and the judge knew or should have known that they were wrong and the harm was so bad it completely tainted the proceedings.
There probably still are some errors that are like that, for example, a judge's coming down from the bench to testify as a witness in a matter. That error would not likely be excused because none of the lawyers objected (They would likely be thunder-struck with disbelief.). Nor would the judge's granting a motion to strike such testimony and instructing the jury to disregard it cure the error.
Incurable error, as a practical matter, likely no longer exists. The granting of a motion for new trial would cure any error. That this is true will tell you how a trial lawyer ought to object to preserve trial error.