! TeX capacity exceeded, sorry [text input levels=15].is caused by nesting your input too deeply. You can provoke it with the trivial (Plain TeX) file input.tex, which contains nothing but:
\input inputIn the real world, you are unlikely to encounter the error with a modern TeX distribution. TeTeX (used to produce the error message above) allows 15 files open for TeX input at any one time, which is improbably huge for a document generated by real human beings.
However, for those improbable (or machine-generated) situations,
some distributions offer the opportunity to adjust the parameter
max_in_open in a configuration file.