I am not much familiar with node js, but I was forced to use node js to launch on heroku.

It all went smoothly, probably until a few days ago, but I didn't know why but it suddenly gives this much error.

Error: /app/index.jade:9
  7| meta(name='viewport', content='target-densitydpi=device-dpi, width=device-width, user-scalable=no, maximum-scale=1, minimum-scale=1')
  8| link(rel='stylesheet', href='/stylesheets/bootstrap.min.css', media='screen')
> 9| link(rel='stylesheet', href='stylesheets/jquery-ui.css', media='screen')   
 10| link(rel='stylesheet', href='stylesheets/jquery.pnotify.default.css', media='screen')
 11| link(rel='stylesheet', href='stylesheets/jquery.pnotify.default.icons.css', media='screen')
 12| link(rel='stylesheet', href='stylesheets/app.css', media='screen') 

 unexpected text at Object.Lexer.fail (/app/node_modules/jade/lib/lexer.js:804:11)
 at Object.Lexer.next (/app/node_modules/jade/lib/lexer.js:863:15)
 at Object.Lexer.lookahead (/app/node_modules/jade/lib/lexer.js:113:46)
 at Parser.lookahead (/app/node_modules/jade/lib/parser.js:111:23)
 at Parser.peek (/app/node_modules/jade/lib/parser.js:88:17)
 at Parser.tag (/app/node_modules/jade/lib/parser.js:704:22)
 at Parser.parseTag (/app/node_modules/jade/lib/parser.js:690:17)
 at Parser.parseExpr (/app/node_modules/jade/lib/parser.js:199:21)
 at Parser.block (/app/node_modules/jade/lib/parser.js:660:25)
 at Parser.tag (/app/node_modules/jade/lib/parser.js:777:26)

This is What my index.jade is about

doctype html
html(lang="en")
  head
    meta(name='apple-mobile-web-app-capable', content='yes')
    meta(name='apple-mobile-web-app-status-bar-style', content='black')
    meta(name='viewport', content='target-densitydpi=device-dpi, width=device-width, user-scalable=no, maximum-scale=1, minimum-scale=1')
    link(rel='stylesheet', href='/stylesheets/bootstrap.min.css', media='screen')
    link(rel='stylesheet', href='stylesheets/jquery-ui.css', media='screen')  
    link(rel='stylesheet', href='stylesheets/jquery.pnotify.default.css', media='screen')
    link(rel='stylesheet', href='stylesheets/jquery.pnotify.default.icons.css', media='screen')

Any suggestion why it becomes wrong?

Try changing your Jade to

doctype html
html(lang="en")
  head
    meta(name='apple-mobile-web-app-capable', content='yes')
    meta(name='apple-mobile-web-app-status-bar-style', content='black')
    meta(name='viewport', content='target-densitydpi=device-dpi, width=device-width, user-scalable=no, maximum-scale=1, minimum-scale=1')
    link(rel='stylesheet', href='/stylesheets/bootstrap.min.css', media='screen')
    link(rel='stylesheet', href='/stylesheets/jquery-ui.css', media='screen')  
    link(rel='stylesheet', href='/stylesheets/jquery.pnotify.default.css', media='screen')
    link(rel='stylesheet', href='/stylesheets/jquery.pnotify.default.icons.css', media='screen')

As @Max Leske said in the comments, you are mixing relative and absolute paths.

Not sure why that would be causing an issue, but it's more than a coincidence I feel.

Incidently, the reason I asked to see your package.json is because recently there was an update to Jade on NPM that has caused the issue you are reporting:

https://github.com/visionmedia/jade/issues/1347

If you have something like:

"jade" : "*"

in your package.json that will basically mean "install the latest version"

So, every time you push to heroku, it will update your version of jade to the latest version, thus introducing any breaking changes.
I suspect this is what happened a few days ago, causing this bug (you pushed to heroku, forcing an update of jade to the latest version)

It would be better to specify version numbers in your package.json file to avoid this.