You run failover cluster validation, and the report claims that one or more of the nodes are missing update KB3005628:
You try running Windows Update, but KB3005628 is not listed as an available update. You try downloading and installing it manually, but the installer quits without installing anything.
KB3005628 is a fix for .Net framework 3.5, correcting a bug in KB2966827 and KB2966828. The problem is that the cluster node in question does not have the .Net framework 3.5 feature installed. It did however have KB2966828 installed. As this is also a .Net 3.5 update, I wonder how it got installed in the first place. After reading more about KB3005628, it seems that KB2966828 could get installed even if .Net framework 3.5.1 is not installed.
So far, no matter what I do the validation report lists KB3005628 as missing on one of the cluster nodes. This may be a bug in the Failover Cluster validator.
If the .Net Framework 3.5 feature is not installed, remove KB2966827 and KB2966828 manually from the affected node if they are installed. The validation report will still list KB3005628 as missing, but as the only function of KB3005628 is to remove KB2966827 and KB2966828 this poses no problem.