Some wide characters (e.g. U+1F201 🈁) really mess up readline. For example, it treats it correctly as a wide character when pasted on the line but then only moves the cursor one point when moving back through the line.
To reproduce, just paste a number of these characters and attempt moving around.. This is what the line looks like after pasting in four of them, going to the beginning of the line and typing echo:
$ 🈁🈁e🈁🈁c🈁🈁h🈁🈁o🈁🈁🈁🈁
This issue is not specific to bash, but in case it’s useful, I noticed that bash does react somewhat strangely to these characters, printing out their escaped version in certain error messages where it would normally print the character.
$ 🈁
bash: $'\360\237\210\201': command not found