I entered a simple cron expression accidentally adding an extra field:
H 0 * * * *

The error message was very misleading:

Invalid input: "H 0 * * * *": line 1:10: expecting EOF, found ' '

Hitting "Save" anyway gave:
Caused by: java.lang.IllegalArgumentException: Failed to instantiate class hudson.triggers.SCMTrigger from

at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:597)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:392)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:388)
at hudson.model.Descriptor.newInstance(Descriptor.java:567)
... 88 more
Caused by: java.lang.IllegalArgumentException: antlr.ANTLRException: Invalid input: "H 0 * * * *": line 1:10: expecting EOF, found ' '
at org.kohsuke.stapler.RequestImpl.invokeConstructor(RequestImpl.java:454)
at org.kohsuke.stapler.RequestImpl.access$200(RequestImpl.java:77)
at org.kohsuke.stapler.RequestImpl$TypePair.convertJSON(RequestImpl.java:595)
... 91 more
Caused by: antlr.ANTLRException: Invalid input: "H 0 * * * *": line 1:10: expecting EOF, found ' '
at hudson.scheduler.CronTabList.create(CronTabList.java:87)
at hudson.scheduler.CronTabList.create(CronTabList.java:73)
at hudson.triggers.Trigger.<init>(Trigger.java:161)
at hudson.triggers.SCMTrigger.<init>(SCMTrigger.java:85)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
at org.kohsuke.stapler.RequestImpl.invokeConstructor(RequestImpl.java:439)
... 93 more
Caused by: line 1:10: expecting EOF, found ' '
at antlr.Parser.match(Parser.java:211)
at hudson.scheduler.CrontabParser.startRule(CrontabParser.java:67)
at hudson.scheduler.CronTab.set(CronTab.java:93)
at hudson.scheduler.CronTab.<init>(CronTab.java:83)
at hudson.scheduler.CronTabList.create(CronTabList.java:85)
... 101 more