Sometimes, we find specific corner cases that cause a failure in our application - for example, broken relationships between the main entities in our application that are not properly handled (that can be reproduced artificially in development environment and have a low probability of happening in production), an extreme or unrealistic value for an input that a regular user would not even consider to enter, problems under certain conditions in a specific timezone with very specific and uncommon steps to reproduce etc.
Most of times, these cases when presented to management and development team are not taken seriously or completely ignored. In some cases, the reasoning for that is perfectly valid and the consequences and symptoms are not critical and are not signs of bigger problems. But, there are also cases, when we, the QA team, think that a specific corner case is a sign of a bigger problem but we cannot prove it.
What can we do to defend a corner case and make it be taken seriously?