First, I think mistakes are inevitable, especially with a card pool the size of Magic's. Adding more testing would not catch every issue. But I agree that adding more testing would catch more issues.
My point was that they are trying to push Standard, and their attempts to do so via new printings (rather than bringing back old staples like Bolt, BoP, StP, etc.) could be expected to have ripple effects in eternal formats even if they were taking seriously their duty to test cards for Standard. [12:40 PM Edit: And they obviously aren't.]
Moreover, Wizard's statements have at least suggested that they don't test for eternal formats at all, both now and in the years past when there were fewer mistakes. If that's true, it would be erroneous to say that the increases in bannings and restrictions stems from them getting sloppy or too relaxed when it comes to testing for eternal formats, because there would be no difference in the amount of testing between the past (0) and the present (0). And if that is correct, it's difficult to tell whether the increase in mistakes in Vintage can be attributed to the failure to adequately test for Standard or simply to the push to increase Standard's power.
Even if they were sufficiently careful such that their attempts to ratchet up the power level of Standard didn't lead to mistakes that require bannings within Standard, the additional synergies and lines of attack available in eternal formats means that any attempt to push Standard's power level makes mistakes in non-Standard formats nearly inevitable.
That would be true even if Wizards did their best to test and QA for those formats, and let's not forget that Mark Rosewater has publicly stated that Wizards does not "have the means to test older formats".
Yes, that is absolutely correct - these cards are not part of anyone's collection, they are only accessible in the preconstructed deck event, and they will not be accessible to anyone after that event ends.
However, as @Aelien pointed out, these cards are not only functional (granted, none of them are super complex) but several of them are also animated, including at least Mox Pearl, Mox Sapphire, Sol Ring, and Black Lotus (not sure about Ancestral Recall). Given that the preconstructed deck event is a short-term promotional event, it seems odd that they would devote animation efforts to these cards unless they have at least an inkling that they may bring them back for a more sustained outing at some point.
In case everyone missed it, there are some real vintage cards - Black Lotus, Moxen, Ancestral Recall, Sol Ring - in the current free Arena event (an "Ashiok versus Elspeth" pre-constructed deck event to promote Theros: Beyond Death). It was quite startling to open a starting hand with Ancestral, Black Lotus, and Underground Sea in the Arena client.
I would not be surprised if they bring a Vintage Cube to Arena at some point in the next year or so, though obviously the reliance on bots would be a weakness.