Before I begin, a word of caution: DO NOT EVER USE THE CONTINUE BUTTON FOR THE CAMPAIGN, SELECT YOUR SAVE MANUALLY FROM THE LOAD GAME OPTION
I beat the game on legendary a few days ago, got about 50 percent completion in terms of audio logs, about 10 hours in game. The next day, I beat the game on LASO, after creating a new save. Originally, when I logged off for a few minutes and relaunched the game, I intended to continue my LASO run, but when I clicked continue, it started me off on what I could best describe as a “mix” of my two game saves (completed legendary run, incomplete LASO run). I closed the game ASAP, then relaunched after hearing from another LASO runner to use the load game option, and not the continue option. Since completing LASO, I have decided to go back on my regular legendary run (again, about 10 hours play time, 50 percent progress). I accidently clicked continue on the menu, however, and I started the game with, once again, a “mix” of my game saves. I had the weapons from my LASO save, as well as the collectibles (which was much less collectibles than on my original legendary save), but the skulls were disabled. I also spawned where my LASO save left off (the area you are left in right after you beat the game with the red lights). I again closed the game ASAP, but when I relaunched it, my original legendary save had been overwritten in a way. The progress was no longer 50 percent with about 10 hours, it was now 31 percent, with 3 hours and 44 minutes, which is exactly the amounts on my LASO save. So now I have two game saves, one with all skulls on, the other without, but with the same exact completion progress. This is about 6 hours worth of collectible grinding gone, because the “continue” button is seemingly cursed. I really would like some way to recover my save, if possible, or for 343 to at least patch this insane bug, because I know if I try to continue from here (31 percent completion), I will accidently click continue again and reset my save once again. It’s already happened twice. The UI in this game continues to astound.
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same my progress was lost aswell. it worked fine playing on both pc and xbox but today i loaded like usual and my character did not load now i cant even launch the campaign anymore so sad. and my save only says i have 0% completion even tho i have some achjievments already
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I’ve also lost nearly all my progress, I’m not very interested in playing knowing this will most likely happen again.
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this just happened to me too 
it was all fine till last night i “continued” but then it took me to the very start although it is strange as it said the game had 12 mins, and the part where the pilot picks me up was done. hopefully something can be done and we can get it back :’)
I also just lost all my progress today. I was on Legendary, last mission. Today when I loaded up it had me at Recovery, which is like the third mission. My upgrades were reset as well. It managed to keep my information about the skulls I found but this is so frustrating.
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I lost all 3 of my files my game was frozen and I restarted my Xbox and when I loaded back on all my saved games were gone and it was just very unfortunate since I was halfway through the campaign on LASO and now if I start a new game it says I have no skulls at all … very frustrating
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Same happened to me. I was loading up Halo infinite on my PC and after struggling with a driver error on my Xbox controller for 30 mins, when my campaign wouldn’t load- it would just count down and then… nothing. I restarted the game then the computer itself, only to reload the game and find a completely overwritten save file back to the first mission with no skulls (even though I had already gotten one). I spent at least 8-9 hours taking my time to explore the side missions in that campaign up to that point and to think that it could very well be permanently gone is a huge bummer.
I wanted to make the best of it, and decided to start a legendary campaign since my last one was heroic, only to have it suddenly drop frames 30 mins into playing and crash. I go to reload and the file is back to beginning cutscene 
Happened to me yesterday. I only had a save game from the first level. I had always used continue, I completed the boss level, it took me back to the map to continue, I had had enough, so turned the console off. The next day I choose to continue, it counts down to zero and nothing.
I also had the controller error message.
I have lost all the hours put in. Very poor, nowhere obvious was it to use the save files and not continue.
Original post is 30 days old and no solution or suggestion. I opened a ticket, but no solution,.
100% Save file with 30+ hours is now gone due to using the continue prompt. I made a ticket. Very upsetting. This is wrong in so many ways. wont play any DLC if i have to start over.
Had my first play on normal overwrite my second play on heroic. Loaded up the normal campaign by mistake. Exit and went to load the other and it was gone.
I just experienced this as well just completed the seuence on legendary with 48% percent completion rate and my brother started playing his normal difficulty save while I was at work and now my save is back in the sequence but with his guns on legendary and with his fob and other open world objectives was his