I don't know if this is one bug or several, but here you go:
⎕UCS 100
d
⎕UCS 1E2
DOMAIN ERROR+
⎕UCS 100
^
⎕ucs ¯1
�
⎕ucs ¯100032+⍳3
ABC
'ABC'=⎕ucs ¯100032+⍳3
0 0 0
⎕ucs 1114112 ⍝ last Unicode char is 1114111
����
