Double prepositions?

首先要說，你問題竟然出現了一個正確的用語”Double Preposition”
Double Preposition中文叫：二重介系詞。
所謂Double Prepositoin，同時用了二個介系詞，如：
A cat rushed out from under the bed，同時出現了from與under，這兩個介詞
如同你的句子一樣，這句在文化上沒有錯
但要留意的是under the bed乃是名詞片語，是from的受詞（注意：preposition的受詞，必定是名詞，動名詞，或名詞相等語）
如：We came from beyond the sea，beyond the sea（海的那一邊）是名詞片語，做from的受詞
同樣道理，from inside the room的inside the room也是名詞片語

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Dear rabbit_cony,
I don't blame you for casting doubt if there is
any grammatical mistake in the notice on the water heater as I can
tell that you are a very seriuos student. It apppears to me that the
sentence in issue is grammatically correct. You seem to have
forgetten that the word ' inside' can be used as a noun, an adjective,
a preposition and an adverb. In this context, ' inside" is not used
as a preposition, " inside the room" is an adverbial phrase used as
the object to the preposition " from ". I truly hope my explanation
could be of a little help to you.