Installing a package over https using apt-transport https causes apt to report that the file size does not match what the server reported. Others have mentioned having this issue on the freenode channels.
Here is some of the output:
W: Size of file /var/cache/apt/archives/partial/wget_1.16-1+deb8u4_i386.deb is not what the server reported 503980 923
W: Size of file /var/cache/apt/archives/partial/iceweasel_52.5.0esr-1~deb8u1_all.deb is not what the server reported 121762 4889
W: Size of file /var/cache/apt/archives/partial/xserver-common_2%3a1.16.4-1+deb8u2_all.deb is not what the server reported 1758568 1719696
W: Size of file /var/cache/apt/archives/partial/xserver-xorg-core_2%3a1.16.4-1+deb8u2_i386.deb is not what the server reported 3129878 3122808
After a little testing I found I could only reproduce this on Jessie. It can be reproduced by installing a package from the mirror using apt-transport-https.
This looks to be the same as a Debian bug that was closed, but does not look to be fixed in Jessie: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802617)