I had two instances of this. The first one actually hurt, the second one not so much. So let's start with the first one.
My "friend" and I were talking consistently for 3 years or so online and we'd even had her over my house several times, spending the night and enjoying each other's company. We shared a lot of secrets and were there for each other. I really enjoyed talking to her and sharing our writing and having fun. The nights where we stayed up pretty much all night together just talking and laughing were some of the most fun nights I've had. I thought everything was going great... Except, she apparently thought I was weird. I learned that she'd talked about me behind my back to a mutual friend. Eventually, she just cut ties with me without any indication she was going to do so. The morning of the mass blocking (Discord, Deviantart, Facebook, even ArchiveofOurOwn), she and I were chatting amiably and everything seemed fine. So I talked to our mutual to find out what was going on. Come to find out, she called me a pedophile. For having a crush on Eridan. A fictional character from Homestuck who was an adult by the end of the series. Who is also an alien. Which clearly makes me a kid diddler. So I was obviously offended and horrified by this behind-my-back accusation. While it hurt a lot, I was glad to be rid of a friend who thought so poorly of me and put on a two-faced front. God knows why she continued to be my friend after the Eridan incident. Little did I know that a fictional alien would be the end of one of my long standing friendships. But it was apparently on false and shaky grounds anyway. :/
Now the one that didn't hurt pretty much at all. I'm pretty sure she was a sociopath or something. She came on really strong after a couple weeks, called me one her best friends (which I was very uncomfortable with because I'm really reserved). She gave me a gift (which I refused several times because I don't like feeling as though I owe people something) but nothing about her really seemed off. Until I saw her in real life, because she lived very close to me. The way she acted to her S.O was pretty off-putting, she was one of those Tumblr style leftists who held views that she didn't fully understand, and was obviously a hypocrite because, despite thinking everyone deserves equal rights and comfort, cheated on her S.O. Multiple times. With many different guys. Despite that her and her S.O had a child together. Eventually, after getting kicked out of her S.O's home and her parents' home (for stealing from them), she went to live with one her male mistresses. She lied to me about not liking him, she lied to me about many things that I don't want to go too deep into.
She stopped talking to me much at all, giving me only one or two word (or emote) responses when I messaged her. Got most of my info from her ex-S.O because she always came crying back to him (luckily, he refused to be manipulated and moved on). She was apparently in an abusive relationship with her male mistress, lying to me about that, too. I couldn't stand feeling like she was trying (and failing) to manipulate me so I sent her a message, letting her know exactly how I feel and how we could salvage what little we had. I was curious to know what she'd say. She left the message on read for 3 months. I gave her so long because I knew she was going through a lot. Then I saw she messaged me out of the blue with "oh, sorry, I forgot you messaged me". Just saw that in my Facebook messenger. Didn't bother clicking on it to read the rest. If she couldn't find time for me and wanted to keep lying to me, I figured she wasn't even worth it. Unfriended her from there and eventually deleted my Facebook.
tl;dr, my first friend talked about me behind my back and said some nasty shit about me. The second friend was a sociopathic liar who thought she could manipulate me like she did with everyone else in her life.