bug#10148: 24.0.91; "pending" regexp Isearch

[Drew Adams]

>      In some cases, adding characters to the regexp in an incremental
>   regexp search can make the cursor move back and start again.  For
>   example, if you have searched for `foo' and you add 
> `\|bar', the cursor
>   backs up in case the first `bar' precedes the first `foo'.

Question: How does this relate to the Isearch error messages that indicate
invalid and incomplete regexp patterns, and are thus more specific:
"\\`Premature \\|\\`Unmatched \\|\\`Invalid "?