This behavior isn't exclusive to parson's chameleons. It sounds like some type of tongue injury due to either missing a shot and injuring it somehow or too little Vitamin A and D3 (poor tongue-eye coordination that led to an injury). Luckily it sounds like it isn't a very serious issue if it only happens on occasion. Though the tongue coming out all the way makes me second guess the seriousness of this (i.e more serious). It's possible that if it's an injury like a sprain, it could already be on the mend. I had an experience a while back with a sprained tongue. I just cup fed my Jackson's chameleon larger feeders and did so infrequently so as to let his tongue heal. Stay away from super small feeders as they increase the risk for the tongue sticking to a surface other than the insect. Put feeders at a close to medium range. Super close and super far distances can be a recipe for disaster with a tongue sprain. Treat it like a strained muscle. You want to use it as infrequently as possible until it can be used in a consistent and accurate manner. However, not using the muscle at all doesn't always help with recovery either. So you definitely want her to use the tongue, but in a safe manner to reduce the risk of injury as much as possible until she heals. Answering the questions below may give us a clue as to why and how she go this injury.
I know you are an experienced keeper, so I'll keep this short:
--What supplements are you giving her? (Include brand and frequency)
--Do you keep her indoors/outdoors?
--Do you cup-feed, hand-feed, or free-range feeders?
--You feed any hornworms?