Wrong length
An IMEI is typically expected to be 15 digits. Short, long, or partially copied values often fail validation immediately.
Length-related issues are common in manual entry, imported spreadsheets, and copied screenshots.
Failed check digit
A value may look correct at a glance but still fail format validation if the final digit does not match the Luhn calculation.
This is one reason sample generators are useful in staging and QA environments.
Formatting and data-cleaning issues
Spaces, hyphens, copied labels, and mixed text can break validation if inputs are not normalized correctly.
Support tools should clearly separate user-facing formatting from backend validation logic.