!$ changes when mistyped?

["Benjamin R. Haskell" <zsh@xxxxxxxxxx>]

Apparently there's something different enough about my work keyboard 
such that I constantly inadvertently type !4 when trying to type !$.  (I 
reaally oughtta bind !$ to a key, considering how frequently I use it.)

The problem, though, is that I often then hit <Tab>, so that I can 
modify whatever !$ was.  When I accidentally type !4<Tab>, it expands to 
'vi .zsh_prompt-'.  But, if I catch my mistake and type C-/ (which 
produces the equivalent of "^_", bound to undo), then delete the '4' and 
replace it with '$', <Tab> expands '!$' to '.zsh_prompt-'.  (the last 
word of command 4 instead of the last word of the previous command.)

Is this behavior expected -- should history expansion/completion 
(terminology?)  change what '!$' references?  If so, is it configurable?

FWIW, my workaround has been to (when I typo, which is frequently):

!4<Tab>
<C-/><Backspace>!$
(ending up with !!$, instead of !$).

--
Best,
Ben