How long ago was this? I'd be surprised to see Claude 3.7 Sonnet make a mistake of this nature.

Either way, when a model starts making dumb mistakes like that these days I start a fresh conversation (to blow away all of the bad tokens in the current one), either with that model or another one.

I often switch from Claude 3.7 Sonnet to o3 or o4-mini these days. I paste in the most recent "good" version of the thing we're working on and prompt from there.