"undefined control sequence" error message in tex file

Question

When I run my .tex, this message appears. But my .pdf file is fine. What can i do?

This is pdfTeX, Version 3.14159265-2.6-1.40.15 (TeX Live 2014/W32TeX) (preloaded format=pdftex)
 restricted \write18 enabled.
entering extended mode
(./final version.tex
! Undefined control sequence.
l.2 \documentclass
                  [12pt]{amsart}
?