Swarmkeeper
Hero
We are talking about a situation where a GM, maybe due being inexperienced, maybe due being in hurry and messing up the math or something like that, thinks that the encounter is far easier than it actually is, thus doesn't frame and telegraph it properly and realises their mistake when the combat has already commenced.
"Oh naughty word, there was a smudge on the module, it said five orcs, not five ogres!"
Got it. The players can still make the determination for the PCs to run. It's not all on the DM to determine "appropriate challenge". It's also on the players to determine how to handle a given challenge. An experienced DM should be able to pivot in the moment without skipping a beat (e.g. the ogres drag the PCs back to their lair to sell to the goblin boss the next day OR the owl bears drag the PCs back to their cave for their soon-to-be hatchlings first meal.) I guess when everyone, DM and players alike, is inexperienced at the table and something like the 5 ogres thing happens, the results might be an unsatisfactory TPK but it is what it is. Adventuring is a dangerous profession. Also, we play and learn. Roll up some new characters and next time the players and DM both know what's going to happen in a similar situation.
TL;DR: Fudging might work when used sparingly in the hands of a skilled DM but it isn't a necessary tool to run the game successfully.