The question is, how is it possible that when manufacturing millions of those CPUs, they ended up with bugs (such the indirect addressing mode) and also, why on earth would you as a manufacturer include unofficial operation codes that may be removed in following revisions?
Why are CPUs any different than software? Companies spend millions of dollars on software development, manufacturing and marketing. Why then would a flawed product get loose?
I mean Apple is a major computer and data company right? How then can one explain how such a big company would—for example—release the iPhone 4 with bad antenna issues? Or heck, release a few seconds of white noise as a Taylor Swift song in Canada?
Your presumption is because a company is big and has checks & balances that those checks and balances should then be flawless. Which is simply not the case in any human endeavor no matter how many layers of quality assurance one has.
In the case of unofficial operation codes, it can easily be explained by rushed development or code being readied for future development. If a new CPU is made, the engineers will most likely have a massive list of desired features. Some will be made. Some will not. And some will be made yet would be impractical to officially stamp as “Yes this is working, uses this.”
This is the reason—for example—why pretty much every CPU out there has revisions. And why in the multi-CPU systems—ones with multiple physical CPUs—it is often important that all CPUs match the same production batch.