Where is the site with a list of OpenStreetMap errors?

There are a few different services that allow you to add bugs, so it's hard to tell which one you were using previously. The most likely answer is that it was OpenStreetBugs

If that's not the case, then you can look through the list of Quality Assurance tools that we have and see if you can find the one you used previously.

Andy Allan pointed to the list of Quality Assurance tools that offer various services that allow to report bugs.

I just want to state that based on my own limited experience, these can be recommended:

• OpenStreetBugs is a nice and easy GUI and, the JOSM editor also features an integration plugin.
• What I find extremly nice done is skobbler.net, which AFAIK integrates with the openstreetbugs database, and has bug reporting integrated in its turn-by-turn navigation. I have not tested their mobile apps though.
• Also worth mentioning: keepright.ipax.at, but that doesn't let you report errors, it generates bug reports automatically, similar to the JOSM validator.

Errors in the Map Data

Quality assurance - Exhaustive list of tools for improving map data quality [Very Long list is provided Please Read]

OpenStreetBugs - A website for easily marking errors in the OSM data.

Mapdust - A QA tool created by Skobbler for easy reporting of issues (bugs) by end users of their OSM-based navigation app. Collected bugs can be reviewed via MapDust.com. If a bug is determined to be caused by OSM data, an OSM editor can fix the data and mark the bug as resolved.

KeepRight - Shows a large number of automatically detected potential errors in the OpenStreetMap map data.

MapRoulette - Shows you one error of a specific type at a time, with a new challenge every so often. Links to the editors to fix the errors.

OSm Inspector

Osmose, whose name means OpenStreetMap Oversight Search Engine, is one of many quality assurance tools available to detect errors and inconsistencies OpenStreetMap data. http://osmose.openstreetmap.fr

Bugs in OSM-related Software Most OSM-related components (the website, and Nominatim, OpenCycleMap, Potlatch, the style of the default map layer) use the same trac. Check the component's wiki page to find the correct place to file a bug if you're unsure.

JOSM and its plugins have its own trac (requires separate account).

Talk:Wiki is a page with suggested technical improvements to the wiki software

WikiProject Cleanup#List of wiki cleanup issues is a list of "bugs" in the wiki content

Duplicate Nodes Map

The Duplicate Nodes Map also uses a simple map interface to show all locations of duplicate nodes.