There was one where she vaulted herself off the couch once, and they were seen. The most obvious time, though (IMO), was when Homer was having his triple bypass surgery, and Lisa arrived in the overhead observation booth, wearing her good pink dress. The camera angle was low and looking up the steps at her, and right up her dress.
You'll find that sort of thing happens all the time in animation (except anime). Dresses and skirts seem to defy the laws of nature, and never blow upwards or drop down, even when hanging upside down. I notice that Lisa is a good example of this on the show.
The weird thing about that, is that Lisa's panties HAVE been exposed many times in the series, and I haven't heard any protests about it. Maybe they are just missing those scenes, and just speed-reading the synopsis of the episodes and then crying about how the series 'have sucked since season blah blah blah', without even watching the episodes in question in any depth.
Sure, some episodes and story-lines have been god-awful or just plain nonsensical, but the majority of the series is enjoyable to watch, and I'm not stopping anytime soon.