The software may be hardcoded to perform perfectly on past data while failing in live conditions.
Decompiling or modifying these files requires reverse-engineering. Hackers alter the code execution paths so the software skips the validation check with the developer's server.
If a standard marketplace EA suddenly requires a custom DLL file to be pasted into your MT5 Libraries folder, it is cracked and highly risky.
A particularly dangerous type of cracked EA is the "trojanized" EA. The code is modified to look like it is trading, but it has hidden instructions to:
Real-world examples paint a grim picture: