That is not a bug. Messages in the so called error log may include items which are not errors. The broad categories that they include are: Errors, Warnings, Deprecated, Strict, and Notice
Only if the message actually says "error' is it possibly an error/bug. Everything else should be considered by non-developers as merely informational. I personally prefer to log everything but strict and notice messages on a production system - as it provides me with a metric to review periodically for potential future issues. I become rather frustrated when reviewing website design to find that error reporting has been disabled entirely because someone was using the number of error messages as some sort of "quality" metric.
