I'll admit it, the rules governing taxonomy and nomenclature can seem horribly complicated when you don't spend a lot of time dealing with them directly. This isn't because taxonomy itself is inherently complicated: in fact, the underlying principles are really quite simple. The primary rationale behind each of the various codes of nomenclature can be distilled down to two points: (a) each single taxon should have a single name that differs from that of any other taxon, and (b) if more than two possible names can be assigned to a single taxon then the name given to that taxon first should be the one used (this latter point is called the principle of priority). Where things get complicated is that taxonomy is a process run by and for human investigators. And if there's one thing that can be said about all human endeavours, from science to politics to the selection of sports teams, it's that any application of simple principles is going to run afoul of complex practicalities. So questions arise that any code of nomenclature has to deal with: is it always ideal to simply use the oldest name? How do we determine which name is 'oldest', and what do we do if it's not clear? Each of the codes has developed its own methods of dealing with these questions and others, but it is not uncommon for these methods to be overlooked or misunderstood, sometimes even by people who might be expected to know better. One particularly pernicious misunderstanding that I've often come across (and which I was reminded of recently by one paper in particular that will go unnamed) is 'page priority'.
As mentioned above, it is not uncommon for two or more names to turn out to be synonymous without one particular name being obviously 'older'. Perhaps the papers naming each species were published at the same time, or they were named within a single paper. In these situations, many authors will invoke the principle of page priority in determining which name should be used: the name which appeared in an earlier place in the publication (say, on p. 23 rather than p. 25) should be the one used. The problem is that, in the case of the International Code of Zoological Nomenclature at least, no such principle is mandated (I'm not so familiar with the other codes, but I don't think they have page priority either). Instead, the code resolves indeterminate priority through the principle of the 'first reviser'. In the code's own words:
24.2.1. When the precedence between names or nomenclatural acts cannot be objectively determined, the precedence is fixed by the action of the first author citing in a published work those names or acts and selecting from them; this author is termed the "First Reviser".
In other words, the question of which name takes priority is determined by the choice of the first person who treats them as synonyms. Any subsequent authors are required to abide by the decision made by the first reviser. There is no restriction placed on how the first reviser should make their decision; if they want to follow page priority, they're perfectly free to do so. The problem comes when a first reviser doesn't follow page priority, only to have later authors claim they made the "wrong" decision. Maybe the name that appeared on a later page in the original publication was better described, or had been more commonly referred to by later authors. Whatever the situation, the decision of the first reviser is final. No correspondence shall be entered into.
"Page priority" is a bit of a case of what's been called a hypercorrection, when someone 'corrects' something that was already right. Someone who's familiar with a principle (in this case the principle of priority) but doesn't fully understand the reasons for the principle may try to apply it more widely than they should. So, for instance, someone who knows the plural of 'hippopotamus' is 'hippopotami' may assume that the plural of 'octopus' is 'octopi'. Hypercorrections persist because, on a superficial level, they 'make sense'. Sadly, being sensible is no barrier to being wrong.
Interestingly enough, there was a brief time in zoological nomenclature when page priority was a mandated rule (Dubois 2010). Between 1948 and 1953, a page priority clause was inserted into the Règles Internationales de la Nomenclature Zoologique, the earlier code of zoological nomenclature that was used before 1961. In 1953, this clause was suppressed and invalidated, and the first reviser principle now applies almost universally. Anyone who argues that a taxonomic decision violates 'page priority' can be safely ignored.
Dubois, A. 2010. Retroactive changes should be introduced in the Code only with great care: problems related to the spellings of nomina. Zootaxa 2426: 1–42.