Try this only in a debugger or if you have another
shell from which you can kill the first one:
zsh% precmd() { echo [[ }
Zsh goes into an infinite loop printing `precmd: bad
pattern: [['. This does not happen in 3.0.8, but does
happen as far back as 3.1.6-dev-20.
Reset errflag after executing precmd
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Fixed in 4.0.2 (patch applies to 4.0.1).
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Argh. Typos in previous comment; it should read:
Fixed in 4.0.3 (patch applies to 4.0.2).
Sorry.