diff --git a/config.toml b/config.toml
index 55568fe..f93447e 100644
--- a/config.toml
+++ b/config.toml
@@ -94,6 +94,9 @@ commands = [
 # `git prole add -b ...` will create a directory named `some-ticket-title`.
 # (The branch name will still be unchanged and way too long.)
 #
+# Note that if the result of applying your regex substitutions includes a `/`,
+# the last component of the result will be used.
+#
 # For completeness, you can also specify how many replacements are performed:
 #
 #     [[add.branch_replacements]]
diff --git a/src/add.rs b/src/add.rs
index 211f73b..a68aba2 100644
--- a/src/add.rs
+++ b/src/add.rs
@@ -119,7 +119,6 @@ impl<'a> WorktreePlan<'a> {
         };
 
         self.git.worktree().add_command(
-            // TODO: What if the destination already exists?
             &self.destination,
             &AddWorktreeOpts {
                 force_branch,
@@ -170,6 +169,14 @@ impl<'a> WorktreePlan<'a> {
     pub fn execute(&self) -> miette::Result<()> {
         let mut command = self.command();
 
+        // Test: `add_destination_exists`
+        if self.destination.exists() {
+            return Err(miette!(
+                "Worktree destination {} already exists",
+                self.destination.display_path_cwd()
+            ));
+        }
+
         tracing::info!("{self}");
         tracing::debug!("{self:#?}");
 
@@ -179,10 +186,11 @@ impl<'a> WorktreePlan<'a> {
                 '$'.if_supports_color(Stream::Stdout, |text| text.green()),
                 Utf8ProgramAndArgs::from(&command)
             );
-        } else {
-            command.status_checked()?;
-            self.copy_untracked()?;
+            return Ok(());
         }
+
+        command.status_checked()?;
+        self.copy_untracked()?;
         self.run_commands()?;
         Ok(())
     }
diff --git a/src/git/worktree/mod.rs b/src/git/worktree/mod.rs
index e437075..7c257fe 100644
--- a/src/git/worktree/mod.rs
+++ b/src/git/worktree/mod.rs
@@ -272,7 +272,20 @@ where
                 }
                 .into_owned();
             }
-            dirname.into()
+
+            if dirname.contains(std::path::MAIN_SEPARATOR_STR) {
+                let final_component = final_component(&dirname);
+                tracing::warn!(
+                    %branch,
+                    after_replacements=%dirname,
+                    using=%final_component,
+                    "Applying `add.branch_replacements` substitutions resulted in a directory name which includes a `{}`",
+                    std::path::MAIN_SEPARATOR_STR,
+                );
+                final_component.to_owned().into()
+            } else {
+                dirname.into()
+            }
         }
     }
 
diff --git a/tests/add_destination_exists.rs b/tests/add_destination_exists.rs
new file mode 100644
index 0000000..b37ff1d
--- /dev/null
+++ b/tests/add_destination_exists.rs
@@ -0,0 +1,21 @@
+use command_error::CommandExt;
+use test_harness::GitProle;
+
+#[test]
+fn add_destination_exists() -> miette::Result<()> {
+    let prole = GitProle::new().unwrap();
+    prole.setup_worktree_repo("my-repo").unwrap();
+
+    prole.sh(r#"
+        cd my-repo || exit
+        mkdir puppy
+    "#)?;
+
+    prole
+        .cd_cmd("my-repo/main")
+        .args(["add", "puppy"])
+        .status_checked()
+        .unwrap_err();
+
+    Ok(())
+}
diff --git a/tests/config_add_branch_replacements_path_separator.rs b/tests/config_add_branch_replacements_path_separator.rs
new file mode 100644
index 0000000..68bec69
--- /dev/null
+++ b/tests/config_add_branch_replacements_path_separator.rs
@@ -0,0 +1,36 @@
+use command_error::CommandExt;
+use test_harness::GitProle;
+use test_harness::WorktreeState;
+
+#[test]
+fn config_add_branch_replacements_path_separator() -> miette::Result<()> {
+    let prole = GitProle::new()?;
+    prole.setup_worktree_repo("my-repo")?;
+    prole.write_config(
+        r#"
+        [[add.branch_replacements]]
+        find = "doggy"
+        replace = "silly"
+        "#,
+    )?;
+
+    prole
+        .cd_cmd("my-repo/main")
+        .args(["add", "-b", "puppy/doggy"])
+        .status_checked()
+        .unwrap();
+
+    prole
+        .repo_state("my-repo")
+        .worktrees([
+            WorktreeState::new_bare(),
+            WorktreeState::new("main").branch("main"),
+            // Last component of the result of the replacements is used:
+            WorktreeState::new("silly")
+                .branch("puppy/doggy")
+                .upstream("main"),
+        ])
+        .assert();
+
+    Ok(())
+}