Strange behavior during error recovery

[Philip Taylor]

Enrico Gregorio wrote:

> If one tries the following test file
>
> \s^^?E
>
> with any plain TeX engine (tex, pdftex, xetex, luatex),
> pressing 1 after the “undefined control sequence”
> error yields
>
> ! Undefined control sequence.
> l.1 \s
>        ^^?H

[snip]

All fine here under Windows 7 Enterprise (64-bit) — the initial "E" does 
not mutate into an "H", and all proceeds normally.

> C:\Users\Philip Taylor_2>tex \relax \s^^?E
> This is TeX, Version 3.14159265 (TeX Live 2020/W32TeX) (preloaded 
> format=tex)
> ! Undefined control sequence.
> <*> \relax \s
>              ^?E
> ? 1
> <*> \relax \s^
>               ?E
> ? q
> OK, entering \batchmode
> C:\Users\Philip Taylor_2>

Philip Taylor

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