A less hostile word that can replace "violation"
Noncompliance should work here.
Compliant:
- Compatible with or following guidelines, specifications, rules, or laws. (Wiktionary)
The browser is standards compliant.
The workplace is compliant with the Americans with Disabilities Act.
EDIT: To further reduce the linguistic baggage, you could tag it noncompliant-code. That way, it's unambiguously clear to the contributors that it's their code that's noncompliant. Not them.
Infraction is often considered less severe than violation. FindLaw:
An infraction, sometimes called a petty offense, is the violation of an administrative regulation, an ordinance, a municipal code, and, in some jurisdictions, a state or local traffic rule. In many states an infraction is not considered a criminal offense and thus not punishable by incarceration. Instead, such jurisdictions treat infractions as civil offenses.
Perhaps nonconformance
a failure to conform to standard norms of behaviour
Collins
Similarly, nonconformity
Failure or refusal to conform to a prevailing rule or practice.
Oxford Dictionaries Online
The latter seems a bit more willful. (But I still like @Tushar Raj's noncompliance)
This is tricky because you are not looking for a general synonym of 'Violation', you are looking for a word that can be used as a Tag against issues that implies the issue is a "violation of agreement, nonconformance to rules and mandatory remedial action required"
I propose "Nonlicensable", being the negative of the adjectival form of the word 'License'
Noun
1 - formal permission from a governmental or other constituted authority to do something, as to carry on some business or profession.
2 - a certificate, tag, plate, etc., giving proof of such permission; official permit: a driver's license.
3 - permission to do or not to do something.
www.dictionary.com
Whilst this is not a synonym of Violation, it does accurately describe the issues you are trying to tag
Oversight
an unintentional failure to notice or do something
I believe making the assumption that the contributor "missed" the rule would be well received. Even if negative intent was there, the code was flagged. It still suggests that something can be done to come into compliance.
In its alternate definition, it means someone else is responsible for reviewing the submission. If an item were flagged under that connotation, it would mean that the responsible party will not assume responsibility for this portion in its current state.