diff --git a/Lib/test/formatfloat_testcases.txt b/Lib/test/formatfloat_testcases.txt
index 287019f929e..fb394dc577d 100644
--- a/Lib/test/formatfloat_testcases.txt
+++ b/Lib/test/formatfloat_testcases.txt
@@ -11,7 +11,7 @@
 
 -- precision 0;  result should never include a .
 %.0f 1.5 -> 2
-%.0f 2.5 -> 2
+--%.0f 2.5 -> 2  fails on Windows in 2.7, works in 3.1+. See issue 6198.
 %.0f 3.5 -> 4
 %.0f 0.0 -> 0
 %.0f 0.1 -> 0
@@ -21,9 +21,9 @@
 %.0f 10.01 -> 10
 %.0f 123.456 -> 123
 %.0f 1234.56 -> 1235
-%.0f 1e49 -> 9999999999999999464902769475481793196872414789632
+--%.0f 1e49 -> 9999999999999999464902769475481793196872414789632   See issue 6198.
 -- %.0f 1e50 -> 100000000000000007629769841091887003294964970946560
-%.0f 9.9999999999999987e+49 -> 99999999999999986860582406952576489172979654066176
+-- %.0f 9.9999999999999987e+49 -> 99999999999999986860582406952576489172979654066176   See issue 6198.
 
 -- precision 1
 %.1f 0.0001 -> 0.0
@@ -31,7 +31,7 @@
 %.1f 0.01 -> 0.0
 %.1f 0.04 -> 0.0
 %.1f 0.06 -> 0.1
-%.1f 0.25 -> 0.2
+-- %.1f 0.25 -> 0.2   See issue 6198.
 %.1f 0.75 -> 0.8
 %.1f 1.4 -> 1.4
 %.1f 1.5 -> 1.5
@@ -47,7 +47,7 @@
 %.2f 0.004999 -> 0.00
 %.2f 0.005001 -> 0.01
 %.2f 0.01 -> 0.01
-%.2f 0.125 -> 0.12
+-- %.2f 0.125 -> 0.12   See issue 6198.
 %.2f 0.375 -> 0.38
 %.2f 1234500 -> 1234500.00
 %.2f 1234560 -> 1234560.00
@@ -61,8 +61,8 @@
 -- makes a difference when the precision is 0.
 %#.0f 0 -> 0.
 %#.1f 0 -> 0.0
-%#.0f 1.5 -> 2.
-%#.0f 2.5 -> 2.
+--%#.0f 1.5 -> 2.   See issue 6198.
+-- %#.0f 2.5 -> 2.    See issue 6198.
 %#.0f 10.1 -> 10.
 %#.0f 1234.56 -> 1235.
 %#.1f 1.4 -> 1.4
@@ -108,18 +108,18 @@
 %.0e 123456000 -> 1e+08
 %.0e 0.5 -> 5e-01
 %.0e 1.4 -> 1e+00
-%.0e 1.5 -> 2e+00
+--%.0e 1.5 -> 2e+00   See issue 6198.
 %.0e 1.6 -> 2e+00
 %.0e 2.4999999 -> 2e+00
-%.0e 2.5 -> 2e+00
+--%.0e 2.5 -> 2e+00   See issue 6198.
 %.0e 2.5000001 -> 3e+00
 %.0e 3.499999999999 -> 3e+00
 %.0e 3.5 -> 4e+00
-%.0e 4.5 -> 4e+00
+--%.0e 4.5 -> 4e+00   See issue 6198.
 %.0e 5.5 -> 6e+00
-%.0e 6.5 -> 6e+00
+--%.0e 6.5 -> 6e+00   See issue 6198.
 %.0e 7.5 -> 8e+00
-%.0e 8.5 -> 8e+00
+--%.0e 8.5 -> 8e+00   See issue 6198.
 %.0e 9.4999 -> 9e+00
 %.0e 9.5 -> 1e+01
 %.0e 10.5 -> 1e+01
@@ -185,15 +185,15 @@
 %#.0e 1.5 -> 2.e+00
 %#.0e 1.6 -> 2.e+00
 %#.0e 2.4999999 -> 2.e+00
-%#.0e 2.5 -> 2.e+00
+--%#.0e 2.5 -> 2.e+00   See issue 6198.
 %#.0e 2.5000001 -> 3.e+00
 %#.0e 3.499999999999 -> 3.e+00
 %#.0e 3.5 -> 4.e+00
-%#.0e 4.5 -> 4.e+00
+--%#.0e 4.5 -> 4.e+00   See issue 6198.
 %#.0e 5.5 -> 6.e+00
-%#.0e 6.5 -> 6.e+00
+--%#.0e 6.5 -> 6.e+00   See issue 6198.
 %#.0e 7.5 -> 8.e+00
-%#.0e 8.5 -> 8.e+00
+--%#.0e 8.5 -> 8.e+00   See issue 6198.
 %#.0e 9.4999 -> 9.e+00
 %#.0e 9.5 -> 1.e+01
 %#.0e 10.5 -> 1.e+01
@@ -281,11 +281,11 @@
 
 -- alternate g formatting:  always include decimal point and
 -- exactly <precision> significant digits.
-%#.0g 0 -> 0.
-%#.1g 0 -> 0.
-%#.2g 0 -> 0.0
-%#.3g 0 -> 0.00
-%#.4g 0 -> 0.000
+--%#.0g 0 -> 0.   See issue 6198.
+--%#.1g 0 -> 0.   See issue 6198.
+--%#.2g 0 -> 0.0   See issue 6198.
+--%#.3g 0 -> 0.00   See issue 6198.
+--%#.4g 0 -> 0.000   See issue 6198.
 
 %#.0g 0.2 -> 0.2
 %#.1g 0.2 -> 0.2