15 years ago, I was using LaTeX on a daily basis for writing my dissertation, articles, and presentations. I used the beamer package to create both slides and notes with the same source for several courses I taught. I am planning on giving a presentation soon and since I am lazy, I wanted to copy/paste an old presentation to get started. I was saddened to discover that the 12 year old LaTeX file I found would not compile, but gave me this strange error.
! Undefined control sequence. l.1 \beamer@endinputifotherversion
I figured that there must have been some backwards incompatible change in LaTeX / beamer in the last 12 years. I tried several other files, and found the same thing. Then I opened up the documentation using texdoc beamer and found mention of an example file included in the package called conference-ornate-20min.en.tex. I tried compiling this example file, and it worked! I also found an old near-copy of the same file called example.tex on my computer. This one would not compile. I started diffing and noticed that the old one used syntax like:
\begin{frame}
\frametitle{title of this slide}
\end{frame}while the newer example file used
\begin{frame}{title of this slide}
\end{frame}I changed all of these in the old file, but it would still not compile. What gives? I decided to get more aggressive, and renamed the old file, and tried again, and suddenly it worked. Huh? It turns out I had some old intermediate files lying around, which LaTeX was trying to use, and apparently the format of these intermediate files has changed in the last 12 years. I tested out this theory with a different file, and also had success. So if you run into weird errors trying to compile old LaTeX documents, make sure to delete all the intermediate files like .toc, .aux, etc. You probably only need the .tex file.
I think it is really remarkable that the current LaTeX and beamer still work with 12 year old files. I don’t think there are many other tools which can boast that sort of backwards compatibility.
Don’t know much about LaTeX? Maybe you would find my article why LaTeX interesting.