Multiple accounts of plagiarism (?) during literature study: are my standards too high or should I take action?
It's also worth noting that conventions on citing and quoting are not universal, and can vary between academic communities.  For example, in mathematics, it is quite common to copy definitions or statements of theorems verbatim, or nearly verbatim, from other papers.  The precise wording and notation can be critical, so wholesale paraphrasing is risky.  However, the convention in this field is that quotation marks are not used in such cases, though a citation should usually appear.  Likewise, if a definition or statement is "common knowledge", it might be that every paper discussing it uses almost identical wording (maybe without citations), even though they have not consciously copied from each other.

(I don't really know how this practice arose, but one point is that it often happens that you want to state a theorem almost verbatim, but modify the notation to match what you're using in the present paper.  Now it's not an exact quote anymore, so you can't strictly use quote marks, and noting your amendments with brackets could be very confusing when mathematical notation is involved.)

This can add up to several paragraphs or pages of "copied" text over the course of a paper.  A newcomer to the field might see this and be horrified by the blatant "plagiarism", but within the field it's regarded as perfectly proper.

Additionally, it's completely normal in mathematics to write things like "The material in this section comes mainly from [3]" without adding [3] to every other line.  It's more about common sense than following rules.

So, the moral is to make sure you're calibrating your standards properly to the specific discipline in which you're working.