This might just be because I have a logger set up, but opening the finishLogin page manually (I had been on the page due to a login error and then clicked log in again so it redirected me there) throws an error since there is no SAML response to parse:

ava.lang.NullPointerException at java.util.Base64$Decoder.decode(Base64.java:549) at org.jenkinsci.plugins.saml.SamlSecurityRealm.doFinishLogin(SamlSecurityRealm.java:258) at java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:627) at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:343) at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:184) at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:117) at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:129) at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58) at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715)

Might be nice to just redirect to / or something, or at least show a more obvious error message.