When specifying a command such as cd dir; somestuff; cd .., cd dir can fail when permissions are lacking, if dir was deleted, or if dir is actually a file. In this case, somestuff will run in the wrong directory and cd .. will thus leave you in the parent directory, which is most certainly not intended. In a loop, this will likely cause the next cd to fail as well, propagating this error and running these commands far away from the intended directories.

Check cds exit status and/or use subshells to limit the effects of cd when it fails.

Bad Practice

for dir in */
do
  cd "$dir"
  convert index.png index.jpg
  cd ..
done

Recommended:

for dir in */
do
  (
  cd "$dir" || exit
  convert index.png index.jpg
  )
done

or

for dir in */
do
  cd "$dir" || exit
  convert index.png index.jpg
  cd ..
done

Exceptions

If you set variables that you can't use in a subshell, you will probably need to use this method. In such a case, you should definitely check the exit status of cd. This will also silence this suggestion.