Never, ever, EVER do a flashback episode again.
Tonight's sacrilegious affair did point out the inconsistency of the long-held canon that Bart was conceived while Homer and Marge were just out of high school (despite, again, as noted in the episode Homer and Marge being near-40 and Bart being perpetual-10).
But that's where the semi-accolades stop.
I know the show has been around almost two decades so it can be a real stretch on the space-time continuum thing... but for those of us who've been with it since nearly the beginning (and those of us to still hold much of the series's mid-90s work to classic, pedestal status) don't mess with the show's rich inner history. It's a betrayal. Please continue with the premise that despite not aging, the characters move forward in time with the rest of us (and the politics, pop culture, celebrities, sensibilities, and technologies of the day). Don't deprive us of Homer being a little kid during Woodstock, his mother being an outlaw for her hippie ways, and Homer and Marge's beautiful late-'70s high school courtship. Among other things.
And while you're at it, please note that Semisonic's "Closing Time" came well after Nirvana. So... using it as background / sweetening music in a montage before Homer apparently invents grunge music doesn't work. However the parodies of Nirvana and Bush songs were nice.
Seriously, though, now that the series is so old that flashing back to just before Bart was born puts the series after the start of its run... just don't do it.