If I accidentally order a food and I loved its taste, can I say the following?
This food came across unbelievably tasty!
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You seem to be mixing up two meanings of "come across". First there is the meaning "discovered by accident":
I came across a wonderful Greek restaurant when walking around the city.
This requires a direct object (come across something). For this meaning to work you need to see something without planning it. The sort of accident when you "accidentally order food" is different.
Secondly there is "come across as" which means how a person's character appears:
The interviewee came across as confident and calm.
In your case the food did not "come across as tasty", it "was tasty". There is an expression you might use:
The food turned out to be unbelievably tasty!
That suggests you did not know whether it would be be tasty or not, but discovered it was tasty.