PHP Notice: Undefined index: start (also: PHP Notice: Undefined index: end)
Presumably, paging doesn't work right on special:contributions under some circumstances
Seen 21 times within 10 minutes, all from wikidata. May be the same user hitting Special:Contributions some uncommon set of parameters (maybe by manipulating the URL?)
A brief look at the code led me to believe that the 'start' and 'end' fields in $this->opts may not be populated when $skip is set.