Added note on user-reported bugs

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+# Known bugs and issues
+
+I will fix these at some point.
+
+## Incomplete filtering when using NISS
+
+When using NISS, sometimes the switch happens when it does not make sense.
+For example, for EO:
+
+```
+nissy solve eoud -a -N -m 5 -M 5 -p "R' U' F R D L' B' L2 U2 R' B U2 B2 R2 B2 D' F2 U D2 R2 D B2 R U R' U' F"
+```
+
+The last 4 EOs returned by this command start with `D2 U (switch...)`:
+
+```
+...
+D2 U (L F D)
+D2 U (L F D')
+D2 U' (L F D)
+D2 U' (L F D')
+```
+
+These EOs should not be included, because the starting `D2` move is useless.
+
+It is possible that this issue affects other steps too.
+
+## Optimal solver duplicate solutions
+
+There is an issue with move ordering that leads to the same solution occasionally being listed
+twice.
+
+To reproduce:
+
+```
+nissy solve optimal -o "R' F' D2 L' U' L B D L2 F2 L' U2 R' F2 R' D2 F2 U2 F'"
+```
+
+Output:
+
+```
+U' D2 F B' U R' U D B R' B' U' L D2 F2 B2 L (17)
+U' D2 F B' U R' U D B R' B' U' L D2 B2 F2 L (17)
+```
+
+The two solutions are the same, but with `F2 B2` swapped.