Question: 2 times an error, on the third try not. Why is that?

I try to understand why Maple throws 2 times an error but on a third attempt (with the same input) output is returned. Is that a new mechanism of suppression of error or warning messages and returning output up to a point where evaluation cannot further be performed.

With -sin instead of cos Maple returns output immediately. Does this mean that there is no information to the user available (yet) for this particular case?

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