ศาลาชุมนุม

[Pirin]

As for the words of Pali and Sanskrit origin, it is said that there are two types of initial consonants appearing in pair.
The first type includes the words whose initial consonants originally come together (such as สยมพร). In Thai, the initial consonants of this type are read according to the rules of cluster.
The second type includes the words whose initial consonants originally appear as separate syllables (such as สมาทาน).
In Thai, their initial cosonants are read as the different syllables (of the same word).

RID:
สมาทาน [สะมา-] ก. รับเอาถือเอาเป็นข้อปฏิบัติ เช่น สมาทานศีล. (ป., ส.).
http://www.thai-language.com/id/32812
สยมพร [สะหยมพอน] น. พิธ๊เลือกคู่ของกษัตริย์สมัยโบราณ, การเลือกผัวของนางกษัตริย์สมัยโบราณ, สยัมพร สยุมพร หรือสวยมพร ก็ใช้. (ป. สยํวร; ส. สวยํวร).
http://www.thai-language.com/id/24499

[Richard Wordingham]

Among the "true clusters" where there is generally no intervening vowel between the first and the second consonant, is ปร- ปล-. So, words like เปรียบ and ปลา are pronounced without an vowel between them. However, there are exceptions, like the word for thermometer: "ปรอท" which is pronounced: /bpaL raawtL/. Note that in this word, the tone of the word is determined by the class of the first consonant (a mid-class consonant) even though the word is pronounced as if it were two syllables.

This is another example of a word taken almost directly from Pali or Sanskrit, as Khun Pirin points out above and where the pronunciation in Thai reflects the spelling as determined by Pali/Sanskrit scholarship. The Royal Institute Dictionary has,

"ปรอท [ปะหฺรอด] . . . (ป., ส. ปารท)."

No way is ปรอท a 'true cluster'. It's an impure cluster. The spelling and pronunciation are a long way off the Pali and Sanskrit antecedents - if indeed the word does derive from Pali or Sanskrit.

The seeming weirdness of having both pure /pr-/ and impure /p-r-/ is a pattern that has recurred in the history of the Tai and related languages.

[Pirin]

The initial consonants of ปรอท are not true cluster because ปรอท was originally composed of two syllables.

When the original long [a] vowel of the first syllable of ปรอท --ปารท-- becomes the short [a] vowel of its first syllable (in Thai -- ปรอท), this word is still read as if it were a word composed of two syllables. In such case, if the initial consonant of the second syllable is a sonorant, its tone is governed by the consonant of the first syllable.

RID:
ปรอท [ปะหรอด] ว. ธาตุลำดับที่ ๘๐ สัญลักษณ์ Hg เป็นโลหะ ลักษณะเป็นของเหลว สีเงิน แข็งตัวที่ -๓๘.๓๖̊ ซ. เดือดที่ -๓๕๗̊ ซ. ใช้ประโยชน์..... (ป., ส. ปารท).

sonorant
http://www.thai-language.com/id/841623

[David and Bui]

Two words which gave me fits are ชนวน and บวร. The difficulty for new readers is distinguishing between ว as a consonant and as a vowel.

ชนวน is pronounced ชะ-นวน [chaH nuaanM] where the ว is a vowel and ชน is a non-conforming consonant cluster. Before I was corrected I thought the pronunciation was ชน-วน [chonM wonM] (See http://www.thai-language.com/id/139795)

บวร is pronounced บอ-วอน [baawM waawnM] and the ว is a consonant. Someone who did not know this word might pronounce it as [buanM]. (See http://www.thai-language.com/id/151460).

So, yet one more barrier to easy learning of Thai orthography.