I found I started making more of those weird errors when I started to use text chat a lot for communicating with business partners. It bleeds through into forum posts as well. Sometimes I will pause writing at a can/can't and then figure out how to write the rest of the sentence. Then I go back and discover the can/can't is wrong. Often I will start the sentence with "can" in mind, then decide on a different wording after the fact that requires a "can't" or vice versa (usually in explaining some kind of mechanical process where either word can be used but I want to find the most intuitive way to say it). Then I will forget to change it until I either send the message or proofread it and find the mistake.