I've been fishing for decades and never had a problem with line breakage until this season.  I'm not talking breaking off at the knot or on a hook set, I'm talking the line breaking  20-30yrd. down.  Today, I saw it on the spool, before a cast and was able to save the lure before casting.  Can a backlash cause a weak spot in the line that would cause breakage during a subsequent cast?  I know the line isn't getting nicked by a bad guide or from banging cover and this was new line I'd just spooled up over the week-end. 

Ya it can. It can cause kinks which can weaken the line.

Yes backlashes can cause weak points in your line and these weak points can break during a cast.

Kinks are especially damaging to fluorocarbon lines. Fluoro is a crystal which will

"crack".

what kind of line are we talking about? flouro is especially prone to damage on a backlash.it can also happen to mono.