This is a perfect example of how legacy spreadsheet functions can quietly fail you, and the explanation is simpler than it seems. The issue isn't that HEX2DEC() is broken or random; it's that the function has a hard limit on the number of characters it can process. Specifically, it accepts a maximum of 10 hexadecimal characters. The value "000003861024" is 12 characters long. That's why removing two leading zeroes to leave three works, you get nine characters total. Removing just one zero leaves ten, which is still within the limit, so why does that fail? Because the resulting string, "00003861024," is also 11 characters. The pattern is consistent: any hex string exceeding 10 characters triggers the #NUM error.
For the user who posted this, the frustration is understandable. You're not doing anything wrong with your data; you're hitting an arbitrary constraint baked into a decades-old function. The real problem is that this limitation forces you to either strip leading zeros manually or build a workaround formula. The user's instinct to convert "000003861024" to "3861024" is the right approach. In practice, you can use a formula like `=HEX2DEC(TEXT(VALUE(A1),"0"))` or simply `=HEX2DEC(--A1)` if the text is purely numeric hex. But the cleanest solution for a column of data is to combine `VALUE()` or `--` with `HEX2DEC()`, which forces Excel to treat the text as a number and drop the leading zeros before the function sees it.
Our take is straightforward: this is a design flaw that doesn't serve modern users. Spreadsheet tools should handle larger hex values without requiring manual pre-processing. The workaround exists, but it's a bandage, not a fix. If you're dealing with long hex strings regularly, consider that a modern AI-native spreadsheet would handle this natively, letting you focus on the analysis rather than the formatting gymnastics. For now, use `=HEX2DEC(TEXT(VALUE(A1),"0"))` and move on, but know that this kind of friction is exactly what progressive tools are designed to eliminate.