Acceptable rate of UI failure depends on the consequences of misinterpretation of the interface. Apparently, for a document uploading service, this case shows

Acceptable rate of UI failure depends on the consequences of misinterpretation of the interface. Apparently, for a document uploading service, this case shows an example that's below the acceptable rate. See it this way; you don't want someone to accidentally trigger a nuclear bomb even once. However you probably don't care that a knife sometimes cuts a finger.