The criticality of an issue is contextual, and can change over time as well as due to framing or extrinsic factors. I describe some silly but illustrative examples below, as well as a couple of more practical considerations of "value."
When is "Critical Bug" Really Critical?
Imagine you have a new phone app, which has a serious bug: every time the user taps the screen inside the app, the user is electrocuted with 200,000 volts of electricity. The organization has labeled this bug as "critical" because electrocuting customers creates bad PR, and reduces the rate of future in-app purchases from customers who are...well, dead.
But what if circumstances change? Can this impact the criticality of the bug from a business point of view?
The Same Bug, Present But No Longer Critical
Reinterpretation as a Feature
Perhaps your marketing department discovers that there's a huge, untapped market in Transylvania by selling the app to the laboratories of a Dr. Frankenstein. You are now selling 1,500 units per week to the good doctor, and your sales are therefore stable. This bug is now considered a "feature" for some sub-set of your customers, so it is re-labeled as a wish-list item to make high-voltage discharges an optional feature rather than an automatic one.
Alternatively, perhaps each copy of the app now sells with an absolutely-free grounding rod and a non-conducting pair of gloves. The PR department says every existing or future customer will get this safety gear "out of an overabundance of concern for customer safety, even though we have zero post-discharge complaints from customers who have personally experienced our market-leading voltage enhancement feature."
Same Issue, Different Criticality
In either case, the issue is deemed less critical because of changed circumstances, a different context, or even plain old marketing spin. The issue is no less real, nor has it gone away. Instead, the value proposition of the issue has changed—perhaps sufficiently that it isn't even considered a bug any longer—due to external or internal factors.
The Value of Features (and Bugs) Change with Time and Lessons Learned
Even aside from the contrived example above, a bug that seems like a show-stopper early on in a project may not remain on the critical path or provide the same level of value later on in the same project. Value changes over time based on a wide variety of factors, and the severity or urgency of any given bug will also change as the value proposition of its related functionality changes, too.
Consider a less silly example where a system has usernames, and the bug is that usernames aren't unique within the system. Is this bug still "critical" if the system moves to using email addresses as account identifiers? What if the application switches to an open, CB-style chat application that doesn't need to retain account information at all?
Bugs are only bugs insofar as they prevent a system from performing a desired function. In other words, a bug subtracts value from a product. In an agile project, the "bugginess" of any issue should change along with the project's evolving notion of value. Your mileage with the WaterFailure™ model will of course vary.