Converting to JSON is a diagnostic, not just a conversion
When a deployment fails on a config file, the error is usually from something several layers away from the parser - a schema validator complaining that a field is the wrong type, or an application reading a key as empty. Converting the YAML to JSON collapses all of that into one view: here is what a parser actually built from your file, with every type it inferred made explicit.
Nine times out of ten the answer is visible immediately. A value you meant as a string is a number. A key you thought was nested is a sibling because the indentation was two spaces where the surrounding block used four. A list is a single string because the dash had no space after it.
The Norway problem, and its relatives
In YAML 1.1, the unquoted tokens yes, no, on, off, y and n are booleans. A list of country codes containing NO for Norway becomes a list containing false. This is the most famous example, and it has a family: 1.20 as a version number becomes the float 1.2 and loses its trailing zero, a time like 12:30 becomes a sexagesimal integer in some parsers, and a value starting with a zero may be read as octal.
The defence is boring and reliable: quote anything that is not obviously a number you want arithmetic on. Version strings, country codes, postcodes, times, identifiers, and anything a human will compare by eye. YAML 1.2 narrowed the boolean rule, but the parser your tooling uses may well not be a 1.2 parser, and the failure is silent either way.
Tabs, and why the error never says tab
YAML forbids tabs for indentation outright. An editor configured to insert tabs, or a block pasted in from somewhere that used them, produces a file that looks correctly aligned and will not parse - and the error typically points at a line some distance from the tab, describing a mapping value that is not allowed here.
If a file looks right and refuses to parse, check for tabs before checking anything else. The second thing to check is trailing whitespace after a colon, and the third is a line that mixes indentation depths within one block. Those three account for most of the parse errors that survive a careful read.