I usually use "That shouldn't be possible" - whether it is possible or if it's user error then often depends on the maturity of the system.
On a new system pretty much anything is possible. On a system battle-tested for years by thousands of users the possibility of encountering program bugs drops dramatically.
This is where good supporters become very valuable. They will be able to learn the solutions to common problems that users face and determine if it´s user error, other errors like OS problems or if it's something new that should be investigated by the developers.
Of course if the bug is reproducible then it's a different matter. But any developer who doesn't take a well-described and reproducible bug report seriously should probable find a different job.