PROBLEM

When executing the following command on OS X…

sed -i 's/megatron/pony/g' /path/to/file.txt

… the following error occurs:-

SOLUTION

Unlike Ubuntu, OS X requires the extension to be explicitly specified. The workaround is to set an empty string:-

sed -i '' 's/megatron/pony/g' /path/to/file.txt

… and now, Megatron has transformed into a pony.