Sourcetree offers the option of stashing your changes before pulling new changes, but if the Stash part of the tree is closed (as it initially is) you may not notice you have to go there. For new users, it would be better if the stash is auto-opened if it was empty, and a new stash is added.

Ideally, the stash entry in the tree, when closed, would show a badge/counter showing how many stashes are in the collapsed tree, which would also give some feedback 'where the stash went'.