Yup. Sometimes when I'm texting back and forth with someone about something and get "interrupted" by a text from someone else, I text that third person back instead of the person I was texting with initially. One night before bed when my husband was out of town, he and I were texting and then a coworker texted me about something. The next morning I replied to the last person I'd been texting with the night before, thinking it was DH. I said "Happy Birthday. I love you!" I didn't realize my mistake until the next time I needed to text this coworker, which wasn't until months later. When I mentioned it to my coworker he claimed that he had assumed I meant it for my husband, but it was awkward because his birthday was actually just a couple days before or after (I don't remember which) so he might have thought it was really meant for him.
Just a couple days ago I was texting with my husband about plans we were setting up with my MIL, while at the same time texting MIL about those same plans. I texted something to my husband that would have been really, really bad if I had sent it to MIL, and for a split second I thought I'd messed it up, but thank goodness I didn't. After that "almost" slip I will be VERY careful in future.