Moral dilemmas and temptations, failures and mistakes, relationship friction, duty and responsibility, power struggles

The character witnessing someone accepting a bribe
Knowing that someone benefited from cronyism or nepotism
Witnessing an abuse of authority (police brutality, evidence being planted, etc.)
Seeing someone skim money from a business 
Watching a certain agenda being pushed via unethical means (biased media coverage, censorship, intimidation, etc.)
Learning a powerful lobby has illegally influenced government officials to pursue legislation that suits its purposes
Discovering an "impartial" company or establishment is owned by an individual or corporation with a clear agenda
Jurors being bought off, intimidated, or influenced
Seeing an obviously guilty party go free because of their wealth, position, or political connections

Being told that what was seen was different than what it appeared
Not knowing who to inform about the corruption 
Being "leaned on" by people who don't want the character to go public with what they've seen
Speaking out and facing unfair reprisals, such as losing a perk or lucrative account
Being investigated by the offending party in an effort to discredit the character 
Having to be available for interviews by reporters, investigators, or detectives
Being "advised" (intimidated) by others to drop the complaint
Nosy neighbors, relatives, and co-workers giving unhelpful advice about what to do
Losing respect for the individual involved (if they were known to the character)
The corrupt party applying a guilt trip about how their life will be ruined if the character says anything
Having to change a routine to avoid the guilty party