Evidence: REFUSED (request rejected; read the body). collapsed HTTP 500 returned 0 row(s) against a RowLimit of 100, labels [], grouping markers [], first row null; expanded HTTP 500 returned 0 row(s); the same query with no <GroupBy> returned HTTP 200 with 100 row(s); ViewXml <View><Query><GroupBy Collapse="TRUE"><FieldRef Name="LVNoSuchColumnAtAll"/></GroupBy></Query><ViewFields><FieldRef Name="FileLeafRef"/><FieldRef Name="LVNumber"/></ViewFields><RowLimit>100</RowLimit></View>; collapsed body {"odata.error":{"code":"-1, Microsoft.SharePoint.Client.UnknownError","message":{"lang":"en-US","value":"Unknown Error"}}}. A group-by naming a column that does not exist did not come back as the flat query, so honoured and ignored are not separable on this run and a served grouped row below says less than it appears to.