When using compatibility mode, it is possible that you will find problems with font-changing commands in some old documents. These problems are of two types:
In case of error messages it is possible that the document (or an old style file used therein) contains references to old internal commands which are no longer defined, see Section 6.2 for more information if this is the case.
One example of the unexpected is if you use one of the new style of math-mode font changing command as follows:
$ \mathbf{xy} A $
You may well find that this behaves as if you had put:
$ \bf {xy} A $
everything including the A coming out bold.
LaTeX 2.09 allowed sites to customize their LaTeX installation,
which resulted in documents producing different results on different
LaTeX installations. LaTeX2e no longer allows so much
customization but, for compatibility with old documents, the local
configuration file latex209.cfg is loaded every time LaTeX2e
enters LaTeX 2.09 compatibility mode.
For example, if your site was customized to use the New Font
Selection Scheme (NFSS) with the oldlfont option, then you can
make LaTeX2e emulate this by creating a latex209.cfg file
containing the commands:
\ExecuteOptions{oldlfont}\RequirePackage{oldlfont}
Similarly, to emulate NFSS with the newlfont option, you can
create a latex209.cfg file containing:
\ExecuteOptions{newlfont}\RequirePackage{newlfont}