Thanks for the reply. What I meant was that the English sentence for the single door should be "There is a narrow door". An "a" is missing in that sentence.
To be more specific:
On 23rd. Nov. 2017 I was asked to "Mark all correct [Polish] translations" of the (incorrect) English sentence
"There is narrow door in your house."
I suspect you may have been discussing a Polish -> English translation exercise.
In the Incubator this is all at one page. The English sentence with Polish translations, and the Polish sentence with English translations... an entry "There is narrow door in your house." does not exist in the Incubator anymore. Which however doesn't stop it from being shown sometimes... :/
Thanks for your insight into your work in the incubator, Jellei :) Now I understand what you meant by 'ghost sentence' - even though I don't understand how it can happen... :/