On client's web site one big problem - i don't understand how of all page generated. It's somehow not use a title field.
For example: http://stankosell.ru/informaciy.html (russian) use keywords in title, but not title.
In which file i can find a bug?
It depends on the extensions you have installed, some will manipulate the title tag. On a vanilla install with no SEO extensions the title tag comes from one of 2 places -