diff --git a/R/guides-.R b/R/guides-.R
index fc9d6e2b3c..67f1ad9373 100644
--- a/R/guides-.R
+++ b/R/guides-.R
@@ -292,16 +292,27 @@ Guides <- ggproto(
     no_guides <- custom
 
     # Extract the non-position scales
-    scales <- scales$non_position_scales()$scales
-    if (length(scales) == 0) {
-      return(no_guides)
-    }
+    scales <- scales$scales
 
     # Ensure a 1:1 mapping between aesthetics and scales
     aesthetics <- lapply(scales, `[[`, "aesthetics")
     scales     <- rep.int(scales, lengths(aesthetics))
     aesthetics <- unlist(aesthetics, recursive = FALSE, use.names = FALSE)
 
+    extra_guides <- setdiff(
+      names(self$guides),
+      c(aesthetics, "custom",
+        "x", "y", "x.sec", "y.sec",
+        "r", "theta", "r.sec", "theta.sec")
+    )
+    if (length(extra_guides) > 0) {
+      cli::cli_warn("Ignoring unknown guide{?s}: {.val {extra_guides}}.")
+    }
+
+    if (length(scales) < 1) {
+      return(no_guides)
+    }
+
     # Setup and train scales
     guides <- self$setup(scales, aesthetics = aesthetics)
     guides$train(scales, labels)
diff --git a/R/layer.R b/R/layer.R
index 8b4621bde2..7a2f2125ee 100644
--- a/R/layer.R
+++ b/R/layer.R
@@ -354,7 +354,7 @@ Layer <- ggproto("Layer", NULL,
 
   compute_statistic = function(self, data, layout) {
     if (empty(data))
-      return(data_frame0())
+      return(data)
 
     self$computed_stat_params <- self$stat$setup_params(data, self$stat_params)
     data <- self$stat$setup_data(data, self$computed_stat_params)
@@ -362,7 +362,7 @@ Layer <- ggproto("Layer", NULL,
   },
 
   map_statistic = function(self, data, plot) {
-    if (empty(data)) return(data_frame0())
+    if (empty(data)) return(data)
 
     # Make sure data columns are converted to correct names. If not done, a
     # column with e.g. a color name will not be found in an after_stat()
@@ -418,7 +418,7 @@ Layer <- ggproto("Layer", NULL,
   },
 
   compute_geom_1 = function(self, data) {
-    if (empty(data)) return(data_frame0())
+    if (empty(data)) return(data)
 
     check_required_aesthetics(
       self$geom$required_aes,
@@ -430,7 +430,7 @@ Layer <- ggproto("Layer", NULL,
   },
 
   compute_position = function(self, data, layout) {
-    if (empty(data)) return(data_frame0())
+    if (empty(data)) return(data)
 
     params <- self$position$setup_params(data)
     data <- self$position$setup_data(data, params)
diff --git a/R/plot-build.R b/R/plot-build.R
index 2a68dd550f..58b13caa62 100644
--- a/R/plot-build.R
+++ b/R/plot-build.R
@@ -102,14 +102,9 @@ ggplot_build.ggplot <- function(plot) {
 
   # Train and map non-position scales and guides
   npscales <- scales$non_position_scales()
-  if (npscales$n() > 0) {
-    lapply(data, npscales$train_df)
-    plot$guides <- plot$guides$build(npscales, plot$layers, plot$labels, data)
-    data <- lapply(data, npscales$map_df)
-  } else {
-    # Only keep custom guides if there are no non-position scales
-    plot$guides <- plot$guides$get_custom()
-  }
+  npscales$train(data)
+  plot$guides <- plot$guides$build(npscales, plot$layers, plot$labels, data)
+  data <- npscales$map(data, layers)
   data <- .expose_data(data)
 
   # Fill in defaults etc.
diff --git a/R/scales-.R b/R/scales-.R
index e62eb0e8cb..24da1cdeaf 100644
--- a/R/scales-.R
+++ b/R/scales-.R
@@ -61,6 +61,13 @@ ScalesList <- ggproto("ScalesList", NULL,
     scale[[1]]
   },
 
+  train = function(self, data, drop = FALSE) {
+    if (length(self$scales) == 0) {
+      return()
+    }
+    lapply(data, self$train_df, drop = drop)
+  },
+
   train_df = function(self, df, drop = FALSE) {
     if (empty(df) || length(self$scales) == 0) {
       return()
@@ -68,6 +75,30 @@ ScalesList <- ggproto("ScalesList", NULL,
     lapply(self$scales, function(scale) scale$train_df(df = df))
   },
 
+  map = function(self, data, layers) {
+    if (length(self$scales) == 0) {
+      return(data)
+    }
+    aesthetics <- lapply(self$scales, `[[`, "aesthetics")
+
+    known <- unique(unlist(c(
+      lapply(data, colnames),
+      lapply(layers, function(x) names(x$computed_mapping))
+    )))
+
+    known <- unlist(aesthetics) %in% known
+    known <- vapply(vec_chop(known, sizes = lengths(aesthetics)), any, logical(1))
+
+    unknown <- unlist(aesthetics[!known])
+    if (length(unknown) > 0) {
+      cli::cli_warn(
+        "Ignoring scale{?s} for unused aesthetics: {.val {unknown}}."
+      )
+    }
+
+    lapply(data, self$map_df)
+  },
+
   map_df = function(self, df) {
     if (empty(df) || length(self$scales) == 0) {
       return(df)
diff --git a/tests/testthat/test-guides.R b/tests/testthat/test-guides.R
index a2e5ae918d..fd69b15d6a 100644
--- a/tests/testthat/test-guides.R
+++ b/tests/testthat/test-guides.R
@@ -95,6 +95,11 @@ test_that("axis_label_overlap_priority always returns the correct number of elem
   expect_setequal(axis_label_priority(100), seq_len(100))
 })
 
+test_that("a warning is generated when declaring unused guides", {
+  plot <- ggplot(mtcars, aes(disp, mpg)) + geom_point() + guides(colour = "legend")
+  expect_warning(ggplot_build(plot), "Ignoring unknown guide")
+})
+
 test_that("a warning is generated when guides are drawn at a location that doesn't make sense", {
   plot <- ggplot(mpg, aes(class, hwy)) +
     geom_point() +
@@ -137,16 +142,13 @@ test_that("guide_none() can be used in non-position scales", {
     geom_point() +
     scale_color_discrete(guide = guide_none())
 
-  built <- ggplot_build(p)
-  plot <- built$plot
-  guides <- guides_list(plot$guides)
-  guides <- guides$build(
-    plot$scales,
-    plot$layers,
-    plot$labels
-  )
+  expect_length(ggplot_build(p)$plot$guides$guides, 0)
+
+  p <- ggplot(mpg, aes(cty, hwy, colour = class)) +
+    geom_point() +
+    guides(colour = guide_none())
 
-  expect_length(guides$guides, 0)
+  expect_length(ggplot_build(p)$plot$guides$guides, 0)
 })
 
 test_that("Using non-position guides for position scales results in an informative error", {
@@ -562,7 +564,12 @@ test_that("legends can be forced to display unrelated geoms", {
       limits = c("A", "B")
     )
 
-  b <- ggplot_build(p)
+  # Should complain about useless scale, as it doesn't map anything
+  expect_warning(
+    b <- ggplot_build(p),
+    "Ignoring scale"
+  )
+
   legend <- b$plot$guides$params[[1]]
 
   expect_equal(
@@ -944,8 +951,8 @@ test_that("guides title and text are positioned correctly", {
     geom_point() +
     # setting the order explicitly removes the risk for failed doppelgangers
     # due to legends switching order
-    guides(shape = guide_legend(order = 1),
-           color = guide_colorbar(order = 2)) +
+    guides(size  = guide_legend(order = 2),
+           color = guide_colorbar(order = 1)) +
     theme_test()
 
   expect_doppelganger("guide title and text positioning and alignment via themes",