LaTeX policy is that things should not change, between releases, in a way that will cause incompatibility with old documents, or will cause typesetting of an unchanged document to change.

Nevertheless, there are problems in LaTeX that it “would be nice” to correct; fixltx2e provides a home for these corrections. Corrections in the current version are:

• ensure one-column floats don't get ahead of two-column floats;
• correct page headers in twocolumn documents;
• stop spaces disappearing in moving arguments;
• allowing \fnysmbol to use text symbols;
• allow the first word after a float to hyphenate;
• \emph can produce caps/small caps text;
• bugs in \setlength and flushbottom.

The package is distributed as part of LaTeX; the extracted package is also available on the archive (location linked here).