Build Failed: One or more errors occurred.

[LarryCarey]

One fine day, after having compiled my project successfully very many times over the preceding weeks, my build failed, and I was presented with the short and sweet status message "Build Failed: One or more errors occurred."
While there may be many possible reasons for this error, here's what fixed my compiler error:
In the Target Editor (HTML5 - Tripane in this case), i changed the Output Folder to the root of my C: drive (something like "C:\Project_Output").
Just in case this saves other noobs from a minor nervous breakdown...

[ChoccieMuffin]

You may have been hit by the 255 character limit somewhere along the line, perhaps when creating a file with a longer than normal name. During the build Flare creates assorted other folders (\Temporary, for example) so even if it looks ok, the temporary build folders can trip you up.