Voiced consonants (б, в, г, д, ж, з) are pronounced as their unvoiced equivalent (п, ф, к, т, ш, з) when they precede an unvoiced consonant, and vice-versa.

For example, the word "ни́зкий" (low) is pronounced "ни́ский", which initially confused me as I couldn't find the word "ниский" in any dictionary!

Voiced consonants are also pronounced unvoiced at the end of words.

For example, the word "по́езд" (a train) is pronounced "по́ест". Don't ever spell it this way though, as it may get confused with the word "пое́ст", which is the 3rd-person singular future tense of the verb "пое́сть", meaning "to eat"!