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

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~!