Politically correct wording for error causes

I am currently making some adjustments to our ticketing system. When the helpdesk closes an issue they have to select what the root cause was. For example a ticket might be created where the user thinks there is something wrong with the product but actually he was not using it properly. I am now looking for politically correct wording of such root causes. Is there a general definition for something like that or some kind of guideline? The following I would like to include:

Human failure
Training issue
Bad design
Poor maintenance
Bad quality

Any input is appreciated! Cheers.

Solution

I suggest to offer the following Error Source Assessment Choices for the user to choose from:

Human behavior = what you call Human failure
Missing or misleading education = what you termed Training issue
Material failure = physical product breaks under certain load, aka Bad quality
Product not fit to purpose = product probably not used as expected by the designers, aka Bad design
Maintenance cycle too long = product is not taken care enough, what you call Poor maintenance
Algorithm needs fine-tuning = the logic behind the IT part of your product behaves unexpected, for those guys and girls who question any number on a display.

I suggest to add the latter point since nearly all (physical) products nowadays come with a software side, even it is only the calibration of the temperature measured by a small resistor.