Is the error in the sentence below a grammatical error that stemmed from a lexical error or vice versa?“It often happens catching myself...

This reads more like grammatical error that stems from lexical error, perhaps because the writer’s/speaker’s native language is not English. The clues here lie in the use of “catching” rather than “catch,” the missing subjective “I,” and the missing conjunction “that” that would create a more familiar English lexical construction. Many non-native speakers, regardless of language, use a general knowledge of vocabulary and verb construction to ‘get by’ in making meaning and being understood. It would be very similar if you were to go to, say, France and ask to use the bathroom or find a restaurant; you would use general vocabulary words you had learned and put them together in a lexical construction that followed patterns of your native language because that it what would be comfortable. This sentence reads like a non-native English speaker using general vocabulary and verb construction knowledge to put together a sentence in a lexicon that is familiar to him or her.

The ending prepositional phrase “in public” is confusing because it is repetitious, not because it is grammatically incorrect. Since public speaking by its very definition occurs in public, it is unnecessary to specify that. If I were to rewrite this sentence, keeping the general lexical construction, I would write, “It often happens that I catch myself feeling insecure regarding public speaking.” This is very wordy, though. In good, concise English construction, the sentence would probably be written, “I often feel insecure about public speaking” or perhaps, “I often feel insecure about speaking in public.”