Why a Space in a URL Can Break Things
A literal, unencoded space in a URL is technically invalid, and different software handles it inconsistently — some browsers silently auto-correct it, while other tools, scripts, or older systems may cut the URL off at the space entirely, treating everything after it as separate, unrelated text, which is exactly the kind of broken-link bug percent-encoding the space as %20 reliably prevents.
This is one of the most common real-world causes of a link that works when clicked casually in a browser but fails when passed through another script, email client, or system.
Why inconsistent handling is the real danger
The unpredictability is the actual problem — a modern browser is often forgiving enough to auto-encode a stray space, masking the underlying issue, but a script, an older application, or a different browser might not be, silently truncating or mangling the URL instead of failing loudly with a clear error.
Why %20 is the reliable fix
As covered in the percent-encoding guide, %20 is the standard, universally recognized encoding for a space, understood consistently by every compliant system — encoding a space this way removes the ambiguity entirely, rather than relying on each individual tool's own guesswork to handle a raw space correctly.