Bug reports and feature requests relating to the MediaWiki software which powers Wikipedia should be filed on Wikimedia's bugzilla. Bugzilla is a bug reporting tool and organization system used by many software projects.
Bugs that are not directly related to the MediaWiki software should be discussed at Wikipedia:Village pump (technical). When in doubt, discuss issues at the village pump before filing a bug on Wikimedia's bugzilla.
Wikimedia's bugzilla was launched in August 2004 to replace the old MediaWiki bug tracker at SourceForge.
You've got a bug, and you're itching to report it. The first thing you should do is search for an existing bug. Someone may have already reported the bug. Searching first helps to prevent duplicates. To be effective, try multiple synonyms and rephrasings of what the bug might have been called.
The next step is to create an account, as accounts on Wikipedia do not carry over into bugzilla. Your email address will be visible on any report you make and on any votes you cast.
Finally, you get to enter your bug! Select the product (usually MediaWiki), and you're ready to go.
Details
Here are what the fields mean (for MediaWiki):
Version - for bugs visible on Wikipedia, choose 1.14-svn
Platform and OS - these usually have no bearing on the actual bug, but it doesn't hurt to leave them specified (if you don't, they are autodetected)
Component - try to figure out what part of MediaWiki the bug is part of.
Severity - how severe the bug is. This should not be an opinion on your part, rather, based on these tags:
Blocker - blocks development and/or testing work (you probably wouldn't know)
Critical - crashes, loss of data (internally, not, say, your edit preview)
Major - major loss of function
Minor - minor loss of function, or other problem where easy workaround is present
Trivial - cosmetic problem like misspelled words or misaligned text
Enhancement - request for enhancement (feature requests)
Assign To - the field will automatically get filled, don't touch it
CC - this field will add people to a mailing list which notifies users when a bug has been changed. It's generally not a good idea to randomly add people to the CC list (let them do it)
URL - a specific URL for the bug, if any
Summary - describe the bug in 60 characters or less. Be pithy, be precise, and be concise. A developer should be able to read the summary and say, "Oh, that's what the bug is about." A bad summary is "Wikipedia isn't working!" Remember, people searching for bugs will often search the summary. Make a good impression: the summary should tell the developer whether or not to look into the bug.
In a description, you should describe anything and everything about the bug. A general template to go about doing this is:
An overall description of the bug
Steps to reproduce
Expected result
Actual result
Additional information
Keep things to one bug to a report, and if necessary, file multiple bug reports. A good bug report is:
Reproducible - if the developer can't reproduce the bug, it'll get marked WORKSFORME or INVALID
Specific - make isolating the bug to an area quicker for the developer
Updates on your bug
You will be emailed with updates on the status of your bug. Sometimes developers may reject, or misunderstand a bug report or feature request and close a report that you think is still valid. If you believe there's still an issue, you can reopen it and try to make a better explanation, or you can take it to the mailing list. If you do not want to get updates on your bug, you can state this in your email preferences.
Voting is a mechanism that allows you to show which bugs you care about. You can vote for a bug by clicking "Vote for this bug" towards the bottom of the page. You can change your mind later and remove your vote by going to the my votes page.
Note that developers are not obligated to implement a request just because it has a large number of votes.
Bugzilla is a place to notify developers of bugs and contribute information that would be useful in the fixing of the bugs. It is not a place for advocacy viz. "I REALLY WANT THIS BUG FIXED!" With that in mind, please keep the following rules in mind when using Bugzilla:
Contribute useful comments; useless comments (i.e. advocacy) decrease the signal to noise ratio
The developers are under no obligation to fix your bug or add a feature you want.
No personal abuse: do not make threats like "I will not edit Wikipedia until this bug is fixed"
Don't mess around with fields unless you are the bug's assignee or you know what you are doing, instead, comment on the suggested change
Don't whine about WONTFIX decisions
Applicability varies, but when in doubt, follow the rules
If you see someone not following these rules, politely mention it to them via private email: do not bugspam some more by chastising the user for their conduct publicly. this is wright bug report
Linking to bugs
You can link to bugs using the bugzilla: interwiki prefix. For example, type [[bugzilla:1000]] for bugzilla:1000.
The mediazilla: prefix can be used for linking to any static page in MediaZilla. For example, type [[mediazilla:index.cgi]] for mediazilla:index.cgi.
Also, note that [[bug:1000]] links to the Buginese Wikipedia, not to bugzilla.