As far as secret endings go, this is one ending that Netflix or 'Black Mirror' creator Charlie Brooker may not have envisaged.
The publishers of 'Choose Your Own Adventure' novels, imaginatively titled Chooseco, have chosen to sue Netflix for trademark infringement, unfair competition, false designation of origin, and trademark dilution. The lawsuit alleges that 'Black Mirror: Bandersnatch' and its use of the phrase "willfully and intentionally to capitalize on viewers’ nostalgia for the original book series from the 1980s and 1990s."
While the concept has - as they rightfully point out - been around since then, it's never been used so widely in a movie. More to the point, 'Black Mirror: Bandersnatch' makes no reference whatsoever to any of their novels, instead creating their own which forms the basis of the interactive movie itself.
Chooseco, who own the rights to the trademark, also claim in their filings that Netflix "actively pursued a license" in 2016, but wasn't granted one. The company are seeking $25 million in damages or profits (whichever is highest) from Netflix.
Neither Charlie Brooker nor Netflix have commented on the lawsuit.