Automatic checkout during a Save operation

It is a good practice to check out files out before you edit them. However, if you edit a file that is under source control but that you have not checked out, the file is silently checked out to your view if, in Eclipse, your Window > Preferences > Team > SCM Adapter > Checked in files are saved by an internal editor preference is currently set to Automatically checkout.

Warning: If you are using a shared branch in a dynamic view, the automatic check out process could create a potential conflict. While you are editing a file that is not checked out, another project team member can check out, edit, and check in the same file before you attempt to save it. When you save the file, the automatic check out process silently checks out the latest version of it and when you check in the file, your edits overwrite the previous edits made by the other team member. To avoid problems, always check out files before editing them.

Related concepts
Checked in files are saved by an internal editor