Why Do I Have a Work Permit Warning on My Visa Sticker?

it is not a problem to have this warning, next time bring your work permit with you. It is a blue booklet, you picked it up at the ministry of labor and has your picture inside, your name and address, the place you work, the job title an description of your job. Example of this booklet in picture.

As you do not know if you presented it or not, I close the commenting.

you did not show the WP3 Application form or the Work permit when you applied for this VIsa? I guess this is your second or more Non immigrant B visa in your passport.