Weatherbase

• weatherbase showing subfreezing temperatures off the coast of California in June (I know there's a cold current there, but ... no.)
• weatherbase claims 68°F in Antarctica in the middle of winter.
• weatherbase has data for a nonexistent town in Maine. (Further investigation based on the latitude and longitude given shows that it's likely to be Portsmouth, New Hampshire).
• Moqiang, an island of cold in southeast China. The city has an elevation of 3199 feet, but this is much too little to result in such extremely cold weather.
• Berbera, Somalia is a manually operated station that often only takes one reading per day, but nevertheless is recorded as a high and low for the day by weatherbase.com.
• Chambéry, France has identical values for "high temperature" and "average temperature" ... so which one is it?
• 9°F in Wales in May. (Nearby stations show temperatures around 30°F).
• 11°F in Scotland in June.
• There's no such place as Plzen, Germany (though there is a Plzen in the Czech Republic).
• Cairo, IL ... not really bad weather data, but the geographical coordinates for this city are quite a bit off ... Google Maps places these coordinates just north of the Tennessee/Mississippi border.
• Anomalous data in San Marino, seemingly lagged one month behind nearby cities such as Perugia and others here.

Note that this section has the most examples of errors because it has the most data ... so I don't believe that weatherbase is the worst of the bunch, by any means. Most of the cities listed above don't even exist in the data sources listed below.

Weather Underground (aka wunderground, WXUG)

• Weather Underground climate data for much of New England shows various stations with surprisingly cool temperatures in summer, even giving inland stations lower temperatures than the coast ... I personally suspect that the cause of this problem is an entire year of data with 32°F for the entire year somehow snuck into the data and was never discovered by whoever was responsible for uploading the data. Note that the anomaly only appears on stations for which the data goes back to 1996. I emailed the Help desk of wunderground.com but never got a response.
• Pierre SD hits 0°F in July 1989 because they were using a thermometer that only had 2 digits. The data nevertheless is recorded by WXUG as a new record low for the month and enters the city's climate data, giving them a monthly average high of 68°F (21 degrees below average).
• Malfunctioning weather station in Sanford, Maine records 21°F on August 3 and an average low of 37°F for the month and is accepted as legitimate data for by WXUG.

Weather.com

• Baltimore is an island of heat, with average summer temperatures exceeding anything you'll find along the rest of the East Coast (or even inland) until you reach Georgia. This heat anomaly is too much to be explained by the station being located in the deep city, which anyhow usually only makes a noticeable difference for night temperatures. All other climate data sources that I can find show Baltimore with temperatures similar to Washington DC.

Paper sources

• Just in case you thought only Internet sources had these problems, the Times Books World Weather Guide has errors of its own, such as recording Monte Carlo with much higher temperatures than its surroundings; and the World Book Encyclopedia (at least the hardcover version) mixes up daily and monthly highs and lows. Extreme weather: a guide & record book by Christopher Burt and Mark Stroud is an example of a book that is filled with lots of interesting information but much of it turns out to be inaccurate. (For this reason, I once removed a claim about weather in Dhahran, Saudi Arabia that used it as its reference; the book itself had simply linked to this page on wunderground.com anyway, which I've addressed above.)

NOAA

Here is a graph of ocean temperatures off the coast of Eastport, Maine, showing them to be far warmer than the surrounding areas, and even warmer than the water off the coast of Northern Florida. Bad readings begin in early October.