Xeno Moral Stains

The term "Xeno Moral Stains" (sometimes shortened to XMS) started as an informal term for circumstances and events in the history of humanity which the Bureau of Xeno Relations recognised as eliciting profound disapproval by alien races, both known and unknown to humanity, within and beyond the Bubble
These were sometimes deduced with difficulty from circumlocutions in diplomatic discussions and sometimes simply and bluntly expressed.
An understanding arose over time that triggers for XMS had little or nothing to do with whatever humanity chose to do to itself. Frankly, the Society Of Contemporary Races had no interest in the numerous atrocities and genocides committed throughout human history. Their attitude to these kind of incidents could perhaps be characterised as, "That's your problem, deal with it." Several of the species known to the BXR, it transpired, were no better than humanity in this respect anyway.
It was only when the actions of humanity caused distress or existential threats to other species that they became concerned and expressed their disapproval. This makes sense, once you appreciate that an ability to live and co-operate with different species is a fundamental requirement for acceptance in a galactic society composed of many different kinds of alien beings. Prejudice against or active harming of "the other" (across species) is worse than "self harm" (within a species) in this context and unfortunately more common.
At this point, it was realised that the most serious examples of XMS were the opposite side of the coin to the redemptive actions of the Genesis Projects and the two concepts became linked in the minds of policy makers.
The three most important XMS from the perspective of the Society Of Contemporary Races are listed below.

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The mismanagement of the Earth's biosphere.

It turns out that the problems humanity had, dealing with climate changes and mass extinctions just before emergence are not especially unusual. The technologies that allow for emergence are typically built on older and dirtier techniques that are necessary intermediate steps and often cause dangerous pollution and ecosystem damage. Nevertheless, the Society Of Contemporary Races likes to see an acceptance of the seriousness of the problems and a properly organised and robust strategy to deal with them. It found Earth's response disappointing before and during the Dislocations and this definitely counts as a significant XMS. Fortunately, the establishment of the Resource Management World Government was viewed positively and has gone some way to mitigating moral disaproval with the state of Earth's ecosystem.

The Escaloda Extinctions

The first extinctions off Earth, for which humanity was undeniably and consciously responsible, were the trigger that turned concern into revulsion and resulted in the Limitation. The events on Escaloda are most likely, still the best known XMS of which humanity stands accused.

The New Cambrian Mirror Struggle

On the world of New Cambria the native ecosystem depends on a chemical model of life, which is superficially similar to the way it works on Earth but uses the opposite chirality for the native amino acids. Colonists from the Republic, by introducing Earth based species have provoked a deadly struggle with "mirror life" which, for whatever reason, (theories differ) the native species appear to be losing. Humanity was explicitly warned about this and its failure to acknowledge the problem or do much about it, is a big XMS.