Data copied from websites, exports and accounting systems often looks normal while containing characters that break comparisons and lookups.
Leading and trailing spaces
TRIM handles ordinary extra spaces, but non-breaking spaces may require SUBSTITUTE or CLEAN-style preprocessing.
Numbers stored as text
A numeric ID and a text ID can look identical while failing exact comparisons. Check with ISNUMBER or ISTEXT before blaming the lookup formula.
Hidden characters
Line breaks and control characters can survive imports. CLEAN can remove many non-printing characters.
Different dash or quote characters
Typography from web pages can introduce visually similar Unicode characters. Normalize them before using exact matching.
Dates that are only text
Displayed date text is not the same as a spreadsheet date serial. Convert deliberately before date arithmetic.