Be me, adding support for new/delete operators in operating system. Works.
Adding iterators and reimplementing array container using templates. Poof, kernel stopped booting.

Why~!?
Bug residing in completely different area of code. Where?

In implementation of reading file from the floppy. Why? Because kernel size with its offset, exceeded 0x0000:0xFFFF. Exactly when I was adding risky feature.

How much time wasted because I thought that it's implementation of templates at fault? Too much~!