Where did the notion that Wikipedia is an unreliable source come from?

Users can modify Wikipedia, but it's not they accept all content. Removes fake and other illegal content. 

the Berlin wall went up at 9 pm on Saturday 12 August and was completed at 0400 on the 13 August wiki even after i corrected it 3 times still says Sunday 13 August making wiki 100% unreliable

From the simple fact that anyone can change the information.

A lot of answers here clearly don’t know much about the Wikipedia platform. People believe that it is an unreliable source because anyone can edit anything. While people can edit things as that is what Wikipedia is all about, the creators can change the editing settings so that posts have to be approved to protect them from vandalism. Look at the source below for more information.

That's the whole point. No board whatsoever. Verification is done by the users themselves.

Wikipedia is as reliable as you can get without buying a carefully edited and researched book on the subject.

People think that just because it is "open source" that it is unreliable.  The thing is that all human activity, including writing textbooks, is unreliable to some degree and one needs to use multiple original sources to corroborate if one wants increased reliability.  As a quick reference for simple information and for most purposes Wiki is both fast and reliable enough.  However it is always a wise thing not to trust any information completely and look for multiple sources to confirm.

Unless you go to a rather obscure page, that could have been made by anyone in their mom's basement.