Here's another one ... and I've only skimmed the PHB so "RTFM" is an acceptable answer.
Is there any rule actually preventing certain daft combinations of races and classes? Can I have a Tiefling Paladin? A Half-Elf Rogue? A Halfling Warlord?
Just imagining a hellborn fiend sticking to the straight and narrow despite being sorely tempted, or a squeaky voice urging his fellows to victory, just makes me smile.
And actually, is a Half-Elf Rogue that daft? What's wrong with a swashbuckling thief, making ladies swoon as he swings out the window? "They seek him here, They seek him there, Those watchmen seek him everywhere. Is he a human? Is he an elf? That damned, elusive Androgelf!"
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)