A couple of times it happened to me but I'm gona mention the last time it did.

Normal 5 v 5. I picked Gragas mid. We had Volibear top, Ashe and Jayce bot, and Malph in the jungle. Malph dc'ed. I went against Kassadin and did what Gragas' do. Suddenly, about 3rd minute of the game our Ashe starts nagging about Jayce taking cs and she says she is gona feed. She went top and got killed by Rene and went mid getting killed by Kass. As we had no jungler she went into the jungler. At the wolves she saw the enemy WW coming out of the blue bush. Forgot to mention she was a total jerk all the time. As she was low on health WW went onto her. Luckily he hadn't his ultimate yet. But I did have mine. And as a good player I am I noticed that, ulted him, pushing him into our tower and allowing Ashe to pick up her first kill. Dunno what happened but she chilled out, I guess, and stayed mid with me for a short time and I let her farm. Our Volibear in the top lane was doing good even tho he lost the tower. He roamed and we picked up a few kills. Our Jayce wasn't doing that good. The enemy team picked up a few kills on him and Ashe early in the game. So I suggested that I and Ashe go bot and Jayce mid. I warded a few spots and Ashe regained strength. Kass, WW and Blitz were putting a lot of pressure on Jayce in mid so we went to help him and soonly an early teamfight begun. We were having a nicely farmed Gragas and Volibear, Malphite AFK, and underleveled Jayce and Ashe. Volibear and me, Gragas, disabled and dealt a lot of damage to the enemy team and Ashe was doing a good job constantly damaging enemies and slowing them. We won that teamfight 4 v 5 with no ppl lost, killing the whole enemy team. And if only that wasn't enough Malph reconnected. I stole a dragon with a barrel, set up a few kills and after so much work we went from 15 kills behind to an even score in about 30 mina. We took baron, just so they wouldn't because most of our lanes were pushed to the inhib. Later in the game we made some mistakes like getting caught solo while warding and stuff. The enemy team pushed to the nexus turrets. Only Ashe and our underlvled Malphite were alive and the enemy had Jayce and Renekton alive both tanky and around 70% of their health. While the enemy minion wave was destroying our nexus, Jayce and Renekton were trying to kill Malph ignoring Ashe the whole time (Ashe was having somewhat complete build) and she melted them both. Finaly I respawned and found our nexus at 20% health. Immeditaly ulting them and clearing them.

Now, I myself have no idea what happened that very moment I ulted the creeps. The destroying nexus animation went on but it wasn't our nexus. Took a look at the chat window and read the enemy team surendering. Why? Dunno. Maybe cuz they didn't have a real carry. Maybe because our inhibs respawned soon after we would have cleared the creeps. Maybe we deserved to win after a such a great turn around. It was an exhausting game and I might upload the replay and post a link.

Other game a similar thing happened was when I played Maokai. We were 3 v 5 having me (Maokai), Olaf and Lux, if I remember. I can't remeber the enemy comp. We weren't doing really bad, just defending all towers we could (with BoM). The other 2 teammates dc'ed around 10th min. We we were 2 or 3 kills behind. The turn around moment was when we cuaght 2 or 3 of the enemy team members killing Drag. We three stole Drake and killed those two or three and the remaining enemy members one by one, pushing the creep waves and eventualy zoning the enemy team inside their base with Lux picking up a few kills. We then stole the enemy buffs and recalled. Warded the whole thing and repeated the whole tactic, Taking down enemies one by one. The enemy team eventually surrendered and we were victorius.

I apologize for any spelling errors cuz I ain't no native eng speaker.