sha1sums / sha256sums / etc files on your download page don't match the binaries.People report this every so often, and usually the reason turns out to be that they've matched up the wrong checksums file with the wrong binaries.
The PuTTY download page contains more than one version of the software. There's a latest release version; there are the development snapshots; and when we're in the run-up to making a release, there are also pre-release builds of the upcoming new version. Each one has its own collection of binaries, and its own collection of checksums files to go with them.
So if you've downloaded the release version of the actual program, you need the release version of the checksums too, otherwise you will see a mismatch. Similarly, the development snapshot binaries go with the development snapshot checksums, and so on. (We've colour-coded the download page in an effort to reduce this confusion a bit.)
Another thing to watch out for: as of 0.71, executables like putty.exe come in two flavours for each platform: the standalone versions on the website, each of which contains embedded help, and the versions installed by the installer, which use a separate help file also in the installer. We provide checksums for both; the latter are indicated with ‘(installer version)’ after the filename.
If you have double-checked all that, and you still think there's a real mismatch, then please send us a report carefully quoting everything relevant: