I will probably upset a few people as this gets into "secret certification business", to misquote an Oz phrase ...
First, a very brief bit of history.
If we go back many decades to when aeroplanes were either little (Tigers etc.) or "big" (DC3, DC4, etc.) the top information (for pilots) document was the Certificate of Airworthiness.
Eventually, it became impracticable for all limitations etc., to be specified in the CofA or placards and a document anciliary to the CofA (the Flight Manual) came into being.
In the US experience this eventually evolved into a Pilots' Operating Handbook (POH) which incorporated both OEM "good guts stuff" (but not approved by the Regulatory Authority) and the Flight Manual information (approved by the Authority).
Later on, the General Aviation Manufacturers' Association (GAMA) published a POH format style for light aircraft which the bulk of such manufacturers has since adopted. I don't recall whether the GAMA format adopted the very similar ICAO Operations Manual recommendations for light aircraft .. or vice versa .. not that it matters all that much, I guess...
Generally, these days, one finds that the ever-present spectre of litigation causes the POH to contain much less than we might desire but that is the nature of life today.
...the often repeated position "follow the POH / manufacturers recommendations".
First, one definitely needs a very good reason to NOT follow the Regulator Approved bits of the POH (the Flight Manual bits) - these are not recommendations - they are instructions. However, there are procedural rules which permit the pilot in command to do just this if the situation so warrants ... but be very prepared to argue your decision at the enquiry ... believe those of us who have been in such positions ... not fun ... no fun at all.
The OEM (non-approved) bits should be viewed as guidance material and not mandatory. The sensible operator will seek to get quasi concurrence from the OEM via a No Technical Objection (NTO) process if it is desired to do something different.
If can paraphrase a checkie from long ago (and, for whom, I had the highest regard) .. "most of these documents leave an important bit out on the front page - to be used with a bit of commonsense ..."
have there been any instances of errors / bad info
You betcha sweet bippy there have been ...
I have picked up numerous such errors over the years ranging from isolated errors (which, when reported to the OEM, usually - but not always - get corrected in subsequent POH revisions) through to my best effort quite some years ago in Oz where I discovered that a particular model of helicopter was flying with the wrong Flight Manual - quite a few helos involved with that one - it was fixed reasonably quickly.
Overall, though, and considering the litigation potential, the POH is subject to a reasonable level of checking and the error rate is pretty low.
Has an accident investigation ever traced ..
Can't bring an example to mind but, I suggest, especially in the avionics/FMS arena, there is plenty of potential.
I commend reading what you can find on the NZ Apia B767 incident ... also a good example of how an airline can proactively and sensibly approach such matters. As an aside, I got a fair bit of privileged information at the time due to my then contract work and I have to say that I was pretty impressed with the Air New Zealand management folk who were involved ....