It would be very helpful if you knew the exact text of the error.

This bug must have an history because I found two other files
(http://commons.wikimedia.org/wiki/File:Alsatia_Superior_Et_Inferior_III_Tabulis_Delineata_,_cum_Finitimis_Episcopatibus,_Ducatibus,_Principatibus,_Et_Marchiona_-_urn-nbn-de-0128-1-11872.jpg
and
http://commons.wikimedia.org/wiki/File:Alsatia_tam_Superior,_quam_Inferior_una_cum_Sundgovia_,_utraque_in_suos_Status_provinciales_divisa_%5E_ex_subsidiis_veteri_-_urn-nbn-de-0128-1-11716.jpg)
which should have disappeared from the Category:Images from Europeana to be
categorized.

Umm, why should they dissapear? That category seems to be included on {{user:Multichill/Europeana}}, so MediaWiki appears to be acting correctly here.

patrick.rump wrote:

No doubt about the help of the error text but please understand that it's too late for me now to remember such details.

What I can add is that this error notice (which wasn't for sure the common connection error notice) asked me to try again and that what I did ; I came then back to the edit page, didn't change anything before submitting and the error occurred again and again.

What is curious too is that when I stopped proceeding (i. e. I didn't ask for a new try and went back to the file page) my changes were done (e. g. http://commons.wikimedia.org/w/index.php?title=File:Cathay_Pacific_Planes_Aligned.jpg&diff=61581147&oldid=43609063). I can't say if I saw these changes immediately or after a while but the files are still listed in the bad category.

But what is funny now is that I just tried to cause this error again whith the ...HKIA.JPG file by submitting again my changes (a dummy change with only the résumé line filled as before) and this file now appears in the specified category without a warning nor a diff record.

I didn't try with the second file (...Aligned.jpg) to let you see if you get the same result and what changes in the database. Maybe this can help you to find out something.

...

About the two last files : I (endly) saw the Category:Images from Europeana to be categorized hidden in the template {{user:Multichill/Europeana}}, that's what actually prevents these files to be considered as categorized even if users did. Since this isn't a bug but a disfunction (and would therefore not be dealt with by you, would it ?), I'll report this problem on the relevant talk page and hope for improvements.

HTH, Papatt

No doubt about the help of the error text but please understand that it's too
late for me now to remember such details.

I just meant that if it happens again, remember the error message. Its totally understandable that you wouldn't be able to remember it now after the fact.

What I can add is that this error notice (which wasn't for sure the common
connection error notice) asked me to try again and that what I did ; I came
then back to the edit page, didn't change anything before submitting and the
error occurred again and again.

Was it the blue error message (Something that looks roughly like http://commons.wikimedia.org/wiki/File:XRomix_%D0%B4%D0%BE%D0%BA%D0%BB%D0%B0%D0%B42008_error_wikimedia.png in terms of colour scheme)? Or did the error message did look like a normal wiki page with an error in it? The blue type of message is usually a php fatal error or connection time out (vs an SQL error)

But what is funny now is that I just tried to cause this error again whith the
...HKIA.JPG file by submitting again my changes (a dummy change with only the
résumé line filled as before) and this file now appears in the specified
category without a warning nor a diff record

That would be consistent with the edit being saved the first time, but some error preventing the categorylinks table from being updated. (see [[WP:NULL]]).

(In reply to comment #4)

I just meant that if it happens again, remember the error message. Its totally
understandable that you wouldn't be able to remember it now after the fact.

Better, if you're going to report it as a bug, copy-and-paste it. I trust that more than my own memory.

patrick.rump wrote:

About error notice :
in my mind it doesn't look like this. It contained about six textlines perhaps
in three sections, I don't remenber coloured stripes but maybe a light
surrounding frame.

About dummy edits :
I agree my bug could come from such an event but the issue is yet abnormal :
the categorization was inefficient (even if it seemed effective, what still
proves the ...Aligned.jpg file) and the end of the process unexpected.

The last question is : why did my last dummy edit put things in order ? Even
did it ? I think so but I'm beginning to doubt. I'd like you to try as I
suggested and await to read your observations.

To Mark :
Please be sure I will. Murphy said it will happen (again). When is another
question.

HTH, Papatt

(In reply to comment #6)

About error notice :
in my mind it doesn't look like this. It contained about six textlines perhaps
in three sections, I don't remenber coloured stripes but maybe a light
surrounding frame.

Look kind of like this one maybe? - http://gyazo.com/4fc4e8a903f977ce43a11d85b54c8b33.png

About dummy edits :
I agree my bug could come from such an event but the issue is yet abnormal :
the categorization was inefficient (even if it seemed effective, what still
proves the ...Aligned.jpg file) and the end of the process unexpected.

The last question is : why did my last dummy edit put things in order ? Even
did it ? I think so but I'm beginning to doubt. I'd like you to try as I
suggested and await to read your observations.

While, at each edit (including null edits which don't get in history page) MediaWiki checks if the article is actually in all of the categories listed in the wikitext of the page, if its not, it adjusts the list of pages that are in Category X to include (or exclude) the current page as appropriate. So if your first edit failed at that step for some abnormal intermitent reason, the null dummy edit would fix it.

patrick.rump wrote:

About notice :
you got it ! I can't be sure for each word but I feel I saw that whole and the line "(SQL query hidden)" speaks to me a lot.

About dummy :
OK, I understand. I'll let the ...Aligned.jpg file as is until you say you won't need to check something by yourself.

HTH, Papatt