diff --git a/NEWS.md b/NEWS.md
index 30b09b9918..010d5c1e22 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,4 +1,6 @@
 # ggplot2 3.1.0.9000
+
+* `geom_hline()`, `geom_vline()`, and `geom_abline()` now throw a warning if the user supplies both an `xintercept`, `yintercept`, or `slope` value and a mapping (@RichardJActon, #2950).
   
 * `scale_shape_identity()` now works correctly with `guide = "legend"` (@malcolmbarrett, #3029)
 
diff --git a/R/geom-abline.r b/R/geom-abline.r
index fc3b1250f1..d309fe3058 100644
--- a/R/geom-abline.r
+++ b/R/geom-abline.r
@@ -82,6 +82,16 @@ geom_abline <- function(mapping = NULL, data = NULL,
 
   # Act like an annotation
   if (!missing(slope) || !missing(intercept)) {
+
+    # Warn if supplied mapping is going to be overwritten
+    if (!missing(mapping)) {
+      warning(paste0("Using `intercept` and/or `slope` with `mapping` may",
+                     " not have the desired result as mapping is overwritten",
+                     " if either of these is specified\n"
+              )
+      )
+    }
+
     if (missing(slope)) slope <- 1
     if (missing(intercept)) intercept <- 0
     n_slopes <- max(length(slope), length(intercept))
diff --git a/R/geom-hline.r b/R/geom-hline.r
index 85bba6e67a..0a689ff7f3 100644
--- a/R/geom-hline.r
+++ b/R/geom-hline.r
@@ -11,6 +11,14 @@ geom_hline <- function(mapping = NULL, data = NULL,
 
   # Act like an annotation
   if (!missing(yintercept)) {
+    # Warn if supplied mapping is going to be overwritten
+    if (!missing(mapping)) {
+      warning(paste0("Using both `yintercept` and `mapping` may not have the",
+                     " desired result as mapping is overwritten if",
+                     " `yintercept` is specified\n"
+              )
+      )
+    }
     data <- new_data_frame(list(yintercept = yintercept))
     mapping <- aes(yintercept = yintercept)
     show.legend <- FALSE
diff --git a/R/geom-vline.r b/R/geom-vline.r
index 3bfd2704c1..8c68a79761 100644
--- a/R/geom-vline.r
+++ b/R/geom-vline.r
@@ -11,6 +11,14 @@ geom_vline <- function(mapping = NULL, data = NULL,
 
   # Act like an annotation
   if (!missing(xintercept)) {
+    # Warn if supplied mapping is going to be overwritten
+    if (!missing(mapping)) {
+      warning(paste0("Using both `xintercept` and `mapping` may not have the",
+                     " desired result as mapping is overwritten if",
+                     " `xintercept` is specified\n"
+              )
+      )
+    }
     data <- new_data_frame(list(xintercept = xintercept))
     mapping <- aes(xintercept = xintercept)
     show.legend <- FALSE
diff --git a/man/borders.Rd b/man/borders.Rd
index 00918d20a6..07544c8cad 100644
--- a/man/borders.Rd
+++ b/man/borders.Rd
@@ -67,7 +67,7 @@ if (require("maps")) {
 ia <- map_data("county", "iowa")
 mid_range <- function(x) mean(range(x))
 seats <- do.call(rbind, lapply(split(ia, ia$subregion), function(d) {
- data.frame(lat = mid_range(d$lat), long = mid_range(d$long), subregion = unique(d$subregion))
+  data.frame(lat = mid_range(d$lat), long = mid_range(d$long), subregion = unique(d$subregion))
 }))
 
 ggplot(ia, aes(long, lat)) +
diff --git a/man/map_data.Rd b/man/map_data.Rd
index a86781457f..ddc28e4bc8 100644
--- a/man/map_data.Rd
+++ b/man/map_data.Rd
@@ -44,6 +44,6 @@ if (require("maps")) {
 ggplot(choro, aes(long, lat)) +
   geom_polygon(aes(group = group, fill = assault / murder)) +
   coord_map("albers",  at0 = 45.5, lat1 = 29.5)
- }
+}
 }
 \keyword{internal}
diff --git a/tests/testthat/test-geom-hline-vline-abline.R b/tests/testthat/test-geom-hline-vline-abline.R
index 58239e8863..83bb42cced 100644
--- a/tests/testthat/test-geom-hline-vline-abline.R
+++ b/tests/testthat/test-geom-hline-vline-abline.R
@@ -43,3 +43,33 @@ test_that("curved lines in map projections", {
     nzmap + coord_map(projection = 'azequalarea', orientation = c(-36.92, 174.6, 0))
   )
 })
+
+# Warning tests ------------------------------------------------------------
+
+test_that("Warning if a supplied mapping is going to be overwritten", {
+
+  expect_warning(
+    geom_vline(xintercept = 3, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    geom_hline(yintercept = 3, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    geom_abline(intercept = 3, aes(colour = colour)),
+    "Using "
+  )
+
+  expect_warning(
+    geom_abline(intercept = 3, slope = 0.5, aes(colour = colour)),
+    "Using "
+  )
+
+  expect_warning(
+    geom_abline(slope=0.5, aes(colour = colour)),
+    "Using "
+  )
+})