This query:

https://query.wikidata.org/#SELECT%20%3Fitem%20%3Fpmcid%20%3Fcites%20WHERE%20%7B%0A%20%20%20%20%3Fitem%20wdt%3AP932%20%3Fpmcid%20.%0A%20%20%20%20%3Fitem%20p%3AP2860%20%3Fscites%20.%0A%20%20%20%20%3Fscites%20ps%3AP2860%20%3Fcites%20.%0A%20%20%20%20%3Fscites%20prov%3AwasDerivedFrom%20%5B%20pr%3AP248%20wd%3AQ229883%20%5D%20.%0A%7D

returns "Server error: Unexpected end of JSON input". I imagine this query has a large number of results, leading to the error, but this error isn't helpful in stating that.

Adding a limit of 100,000 results does prevent this error, but it would be nice to be able to get all of the results (or be told explicitly that I can't get them).