Is 突き挿す misspelling of 突き刺す or alternative writing of it (it is not listed in EDICT or on dictionary.goo.ne.jp) or is it completely different word?
突く is "to prick/push", 刺す is "to pierce/stab" and 挿す is "to insert". While 突き刺す is a common combination, 突き挿す is at least not common, and my IME does not convert つきさす to 突き挿す.
But that does not mean you can call it a misspelling without even seeing the context. There is no such thing as the comprehensive list of compound verbs, and native speakers may combine verbs if they feel it's natural. To me, 突き刺す sounds quick, sharp and violent, while 突き挿す sounds slower and duller, like pressing your finger on clay. I don't remember if I have seen 突き挿す somewhere before, but I feel it makes perfect sense in some situations.