When package source tarball name is different than package name itself, and alternate variable name, different than $pkgname, is used in sources=(), the URL of package in AUR web frontend is incorrect, because it expects exactly $pkgname in source URL.

For example, this PKGBUILD fragment:
...
pkgname=python-foo
pkgname_=foo
pkgver=1.2
source=('http://foo.org/$pkgname_-$pkgver.tar.gz')
...

instead of pointing to http://foo.org/foo-1.2.tar.gz links to http://foo.org/python-foo_-1.2.tar.gz