Not mistakes I’ve made but that I hear others sometimes make:
“Yagdil torah veya’adir” (also in speech, “lehagdil torah ul’ha’adiroh”). There is no patoch under the alef. It’s “veyadir” and “ulhadiroh”.
“Yir’u es Hashem kedoshov”, should be “y’ru”.
“Nachaloh ovar al nafsheinu” means an inheritance; the correct word is “nachloh”, a stream.
Everyone is aware of the rule for a patoch under a ches at the end of a word (i.e. “Noach” rather than “Nocha”). Many seem unaware that the same rule applies to hei and ayin. For instance people say “gavoha” when they should say “gavoah” (pronouncing the hey because it has a dot in it).
And here’s one I used to mispronounce for many years, not in davening but in Tehillim: I used to say “ve’esh nish’koh beyaacov”, thinking it meant something like the fire “kissed”, i.e. singed. Only a few years ago did I realize it’s “nis’koh” as if it were spelled with a samech, meaning “burned”.