Metathesis is when two letters switch places. It’s quite common even in English as you can see here. In Hebrew, it happens in the hitpael forms. Whenever the הת prefix is followed by a letter with the “s” sound (or a sibilant), the ת of the prefix and the sibilant change places. Thus the Hitpael of
שָׁמַל is הִשְׁתַּמֵּר
Notice that the ת switched places with the שׁ. video