Hmm...ever since commissions came out I stopped having that issue with my story mode missions (kinda hard to fail a commission, I'm just saying) but when it comes to repeating events for those silvers or doing the challenge missions, I have had that issue every so often.

That being said, that occurs primarily because I didn't follow my original deployment/skill pattern to a T, and come to find out the consequences of it as the mission rolls along.

Not following your routine of success is an overlooked and fatal mistake. You put an archer in the wrong slot, a demon ignoring terrain passes through and decapitates her in seconds. You forget to flip on the defense buff skill on your tank, suddenly 3 mobs smacking away at the shield turns from sticks and stones into "terrible, terrible damage". You deploy your prince too late and fail to block the way of a mob, and next thing you know he's turning around and picking up a freshly cut-down Phyllis who he just spent the other night giving a new reason to live to...not that it matters anymore...

For easier missions, you have a number of tactical mistakes you can make before everything goes to hell. This is generally determined by the strength of your entire loadout and its functionality as a team. The stronger your team, the more tactical mistakes you can make. The...less developed, the fewer you can handle and the higher the need to stick to a known success pattern.

Harder missions frequently become less forgiving due to unforeseen circumstances. Deploying your units too early or too late can be in themselves a drastic mistake, and as a result cause casualties seconds to minutes later.

A more specific example is facing off against those oversized mummies in story quests and some events: mummies deal damage based on how low their hp bar is at. Thus, if you space out the attacks for a mummy to weak-medium slap whatever unit he's up against, then the damage can be manageable. Push up the hurt on him too fast and he may end up making a blow that your unit couldn't survive on full hp. Even if you have healers on standby and ready to recover that unit to full, the timing may be poor if you have other units being engaged at that time as well.

Any case, yes, it happens to just about everyone every so often, more so depending on your layout and how flexible your deployment strategy is to that particular mission. That being said, that issue is always OUR mistake, pretty much never an unexpected incident on the CPU side where the mobs just happened to be different in some way.

My biggest derp: multi-gaming and as a result failing to deploy my units 20 seconds after the mission has started.