Hundreds of workers received unusable W-2 forms from an employer who should know better. They work for the Internal Revenue Service’s regional center in Fresno, where most Californians send their federal tax returns. The W-2 forms had printing that was illegible or figures that were inaccurate, said Barbara Mecca, an IRS spokeswoman. She blamed a processing center in Detroit. Mecca said the federal agency has replaced most of the defective forms. She said the IRS employees will not get an extension.