Wikipedia's upgrade to version 1.5 of the MediaWiki software continued this week as developers finished converting the remaining projects and moved on to fixing newly discovered problems. The bug that undoubtedly affected the most people was a temporary glitch in the operation of user watchlists.

Editing on Wikipedia after the upgrade to MediaWiki 1.5 began last Tuesday after a day of read-only service to accomplish the transition (see archived story). Once the English Wikipedia was finished, the developers moved on to upgrade the remaining Wikipedia languages, along with Wikibooks, the one project that had not been converted to version 1.5 before work began on Wikipedia. For the most part, this was accomplished with less inconvenience than the day that the English Wikipedia had to be put in read-only mode. Chief Technical Officer Brion Vibber noted that the second-largest project, the German Wikipedia, was not too large to be upgraded overnight, and was fortunate "to have an 'overnight' in the first place."

Since Wikipedia and other projects of the Wikimedia Foundation are used for beta testing the software, to a certain extent bugginess is to be expected at the beginning of an upgrade cycle. A number of problems were listed on a page for MediaWiki 1.5 bugs, although the MediaZilla bug tracker is the place where bugs are actually reported and monitored.

At some point around 12:00 (UTC) on Thursday, 1 July, the watchlist feature broke down and no longer displayed new changes to pages being watched. This is a serious problem, since as Carbonite pointed out, "It takes a lot longer to revert vandalism without current watchlists." During this time, vandalism was reported on the Main Pages of both the English and German Wikipedias.

The effect of this bug was that if the most recent edit to a watched page came after the breakdown, the user's watchlist would no longer show edits to that page. Earlier edits to watched pages would still show up as long as they were still the most recent and no newer edits had been made. As a temporary workaround, Nohat mentioned that it was possible to copy one's entire watchlist and create a user page with all of these links. Clicking on "Related changes" would then reproduce the same behavior as a normal watchlist.

The reason for the problem was finally tracked down as Vibber reported, "This was the result of a bad interaction between an experimental database optimization in the watchlist, and a fix for a minor issue in recentchanges which broke the assumptions the watchlist optimization hack was based on." Watchlists disappeared entirely for a brief period as the developers tried to resolve the issue. Eventually a solution was found, which allowed older edits of watched pages to be seen again, along with edits coming after the fix. However, watchlists apparently will not be able to show edits that happened between the time of the initial glitch and the time it was repaired.