There are plenty of environments where deviating from spec (even when fixing an apparently trivial bug) would not be ok without assessing the situation for potential unintended behaviors.

High-risk systems where a software bug could result in loss of life - aviation, submarine tech, defense, medical etc. Edit: This is for throwing exceptions on undefined behavior.

For unlimited threads I imagine scenarios exist particularly when scalable computing is so popular. Large high traffic web stores such as Amazon/Ebay, financial institutions, etc. Either way, the problem shouldn't be solved by an individual developer on a project making things up as they go - its a problem that probably needs to be defined at a framework layer, and discussed within the team of developers and requirements definers.