Automatic conversion is fast. It is also literal. These are the problems people hit most often.
One Simplified form, several Traditional forms
后, 发 and 面 are famous examples. The converter picks from dictionaries and context windows. It can still choose the wrong Traditional character in a name or a classical quote. See one Simplified character, several Traditional forms.
The file was already broken
If the source is mojibake, conversion multiplies the mess. Fix garbled text first.
Mixed Japanese or Korean
Kanji look related. They are not a drop-in OpenCC input. Convert only the Chinese portions, or you will damage Japanese text.
Regional words
A Taiwan phrase table will not make Hong Kong readers feel at home, and the reverse is also true. Choose the right OpenCC type.
Layout
Traditional text can wrap differently in slides and table cells. That is a layout check, not always a conversion bug.
Work through the proofreading guide after any important job.