This happens in 2012.11, I dunno about the older versions.

I noticed this with a few of my packages, but most recently with a custom kernel tar file url.

For example, I have the following URL in my .config file:

BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION="https://github.com/raspberrypi/linux/archive/rpi-3.6.y.tar.gz"

While make-ing, buildroot adds an extra slash to the URL and tries to download  "https://github.com/raspberrypi/linux/archive//rpi-3.6.y.tar.gz" instead; this results in an error (at least if the target server does some redirection kung-fu like github does).

Example Output:

>>> linux custom Downloading
--2013-01-04 01:30:29--  https://github.com/raspberrypi/linux/archive//rpi-3.6.y.tar.gz
Resolving github.com... 207.97.227.239
Connecting to github.com|207.97.227.239|:443... connected.
HTTP request sent, awaiting response... 302 Found
Location: https://nodeload.github.com/raspberrypi/linux/tar.gz//rpi-3.6.y [following]
--2013-01-04 01:30:30--  https://nodeload.github.com/raspberrypi/linux/tar.gz//rpi-3.6.y
Resolving nodeload.github.com... 207.97.227.252
Connecting to nodeload.github.com|207.97.227.252|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2013-01-04 01:30:30 ERROR 404: Not Found.

--2013-01-04 01:30:30--  http://sources.buildroot.net//rpi-3.6.y.tar.gz
Resolving sources.buildroot.net... 176.9.16.109
Connecting to sources.buildroot.net|176.9.16.109|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2013-01-04 01:30:31 ERROR 404: Not Found.

make: *** [/home/gamaral/development/mm-buildroot/output/build/linux-custom/.stamp_downloaded] Error 1