Because OpenStreetMap is a volunteer project it is never complete, this means that there are areas which are not complete or have errors.
Common errors in sparsely mapped areas include no road names or objects in the wrong places, but there are also likely to be problems in more densely mapped areas either due to mistakes or due to the world changing and the map not yet being updated.
The basic idea of the open system of OSM is that anyone can correct mistakes themselves (for an introduction to this, see the Beginners' guide).
However, even if you only want to point out a mistake then that help is very welcome. There are a few different ways to do this but it may take some time before someone makes the fix.
Notes can be added by anyone, whether they are registered or not. They allow a discussion between users to assist with mapping or editing.
Notes allow for two way communication, with the ability for a mapper to ask for more details if necessary. Although it is not necessary to be logged in to submit (or comment on) a note, it is beneficial to do so. Logged-in users will receive an email if comments are added to a note or the issue is resolved.
The fixme=* tag is used on items in the OSM database to show that there is incomplete or incorrect information.
The advantage of this method is that it is marked on the object itself and so will show up in every editor but the disadvantage is that it is less visible without using other tools.
Other ways of reporting errors
If you do not want to fix the error yourself, or you are not sure how to, then the best option could be to contact other mappers for advice. See contact for various ways of contacting people.