When using underscores within folder names it can sometimes cause ambiguity and lead to broken URLs on your website. The reason is that, for example, a folder named u_2 (after the band, of course) can also be parsed as an attempt to "underlay" a resource named "2" over the base image.
To be on the safe side you should be sure to obey to one of the following:
- Avoid using underscores "_" within names you give to your folders. You can use dash "-" as a delimiter instead.
- Use at least 4 characters prior to the first underscore occurrence, e.g. abcd_1234.
- Use versions to isolate the public ID from the transformation section.