Recompilation process resulted in errors even when I didn't change the Source code which I got after decompiling the WhatsApp APK.
What's the reason for the errors?
The decompilation process isn't perfect and the code you get won't mirror the original code 100%. Things like variable names, loop structures and anonymous inner classes might be interpreted differently and not compile correctly unless manually adjusted.
Source: Decompile Android
For more information take a look at this SO answer about recompiling or this about decompiling.