I unzipped extension into wrong folder. I had ext/ext name/ext/author/ext name/...instead of ext/author/ext name/...Problem is that even when extension was in wrong folder I could see extension in ACP, I could see details about it and it allow me to install it and nothing triggered error. After installation my test forum stop working and I could see on every page only error message that some file is missing. It Would be great if user see some kind of warning that extension is in wrong place before installation (some important files were missing) or if forum disable such wrongly installed extension automatically and let user to uninstall it in the ACP.
I used phpBB 3.1 A1 installed on localhost and extension "Precise similar topics" found here: https://github.com/VSEphpbb/precise-similar-topics-II/tree/extension.