I've had Aztec Magic Megaways from BGaming close prematurely during the freespins. There was 1 spin to go, and the middle 4 were absolutely packed with the sticky wilds - quite a few of them 2x and 3x as well. I was already at 600x and had a real monster spin incoming.
I was only credited the 600x but didn't take it any further because I figured they'd be just as likely to void the whole round as to honor that missing spin. And looking at comments above, that might well be how it would have ended.
On Savanna Roar from Yggdrassil, the game stopped working during a respin feature which sometimes will go on untill it lands the free spins. Leading up to that, it had been paying really, really well for a couple of days: a 1,300x here, an 800x there, free spins coming in easily and so on. I couldn't access the game for a whole week after that, and when I did, there were no trace of the spin that was interrupted. This game is buggy as hell, though. I got 8 ways of the top symbol all across and it payed 4x, while certain features will overwrite a better winning combination. Also, sometimes it plays the "big win" fanfare before entering the free spins, revealing the amount you've won before you've even started it! ?
When I started playing Bermuda Riches, must be about 2 years ago, there was a whole week were it just wouldn't stop paying. Then, over the following week, it couldn't make it past the loading screen even though it was working for other players. The casino support could see no issue whatsoever. Very strange.
Something similar happened just this month. I've enjoyed some good wins on the Midas Golden Touch game from Thunderkick. This game has always been running smoothly ans uninterrupted.but after a 250 spins round with 5 free spins landing, all of them paying real well, soon enough, the dreaded "an error has occured" message popped up. I don't think I've seen even a 30x after that...