iterm – Two red lines / bars in the iTerm2 window and end of session error message. What's going on here?

I experimented yesterday with threading and running iTerm Python bits in MacOS. Today I opened iTerm and two windows open, one with thick red lines.
I have received a warning message from MacOS:

A session ended very soon after the start. Check if the command in the profile "Default" is correct.

I said OK, the window with the red window closed immediately and I finished iTerm.

Question: What is really going on here? I've never seen red lines appear in iTerm2 or a session immediately after launch. Is this just "one of those things" or could something seriously be wrong that requires attention?

I have only one profile, which is called standard.

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