What's the messiest legacy data format you've ever had to work with?

Curious to hear from others in this space — what's the worst legacy data situation you've encountered? Multi-sheet Excel files with merged cells and inconsistent headers? Scanned PDFs with terrible OCR quality? Old fixed-width text exports with no documentation?

I'll go first: I once worked with an export where dates were stored in three different formats within the same column, depending on which year the record was created. Took longer to figure out the pattern than to actually clean it.

Would love to hear what others have run into — and how you approached fixing it.