Filenames, long paths, and special characters that quietly won't sync

Dropbox stores files in a server namespace that is far more permissive than the local filesystems it mirrors them onto. On Windows and macOS the slash characters are always disallowed, the characters colon, asterisk, question mark, double-quote, pipe, less-than, and greater-than are 'not recommended' and cause sync or display problems, and a name ending in a period is illegal on Windows. When the Windows desktop app encounters a restricted character it silently replaces it with an underscore, altering the filename without asking.

Windows also enforces a legacy 260-character maximum path length (MAX_PATH). While modern Dropbox clients can read and write paths beyond that limit, other applications — including older versions of Excel — still fail at the OS limit even after Dropbox has synced the file, and historically over-length paths could fail to sync at all without an obvious error. Emoji are officially supported but typically count as two characters against the 260-character budget, pushing long paths over the limit faster. A 2014 developer write-up documented Dropbox mishandling paths longer than 260 characters on Windows.

The net effect is a class of files that look fine in one place but silently fail to appear, or appear renamed, in another.