I've been thinking about this a bit as I start to look at the huge list of bugs I should probably fix on my game.

What are the worst types?

To me the absolute worst type I can imagine is a late game bug in a longggg game (RPG, adventure, whatever) that either makes it impossible to move forward or corrupts or deletes your save file in some way. I remember both Twilight Princess and one of the Metroid Prime games (2 I think?) had bugs like this. Nasty! Luckily I have none of these type of bugs (not that I know of, anyway.)

Another one that is bad for me is anything that ruins the framerate and makes things hard to play normally. That isn't strictly a bug but it definitely CAN be. (For instance, in my game when parts get super bad framerate issues it is usually not because I'm pushing the system too much but more because I messed up the code that handles spawning and spawned way too much off-screen stuff at once...)

Obviously the frequency of the bug matters too. A super rare bug that happens say, one out of every 10,000 playthroughs won't be as big of a deal as one that happens every playthrough.

Anyway, what do you think? What kind of bugs ruin your experience the most?

And on the contrary, which ones are more harmless and fun? To me stuff like getting on the roof and finding the "secret" room in Super Mario 64 can hardly be considered a serious issue, it's almost like a neat unplanned easter egg. It might not even be a bad thing to leave these kind of "bugs" in on purpose.

My answers are the same as yours, except I don't care of that save deleter at any phase in the game. Short game, medium game, long game... Those suck. Anything that just causes a lock in progress or corruption sucks even if I'm only past the tutorial stage.

Bugs that are exploits and allow unfairness also bug me. See, Snaking and certain shortcuts in Mario Kart games.

Worst: The game ending bug in SNK vs. Capcom: Card Fighters DS for the Nintendo DS (obviously). Talking to a necessary NPC crashed the game halfway through the adventure with no hope of bypassing it. What a waste of time/money.

Most Fun: The Negative World in Super Mario Bros. of course! (or “Minus World” if you are anyone besides Andrew)

Most Irrelevant: The sound glitch in the 3DS version of Mega Man Legacy Collection. ...see? You didn’t even realize it was there!

The worst common glitch for me is the game freezing and me losing progress (more commonly since the last save, but the worst is losing the full file).

The most fun glitch has gotta be in the original (non-DX, non-VC) version of Link's Awakening! The screen skip glitch allowed for a ton of alternate types of game runs, and I would commonly challenge myself to see how many items and upgrades I could get before nabbing the sword. It's surprisingly doable, especially if you stumble into the Fire Rod early. It also taught me a bit about how maps fitted together in those early Zelda games. Definitely my favorite glitch.

The most fun glitches are the ones that make the world bigger and add depth to a game. I know Mario 64 like the back of my hand, but the whole A Button Challenge saga has added a HUGE new dimension to the game, and it's fascinating.

Since becoming a game developer myself, all bugs are the worst bugs and I hate them in every game now.

@Secret_Tunnel
I agree! Metroid has my favorite glitches. You can just bust your way into adjacent areas, even ones that didn't make it into the game proper!

Mario Kart glitches are the freakin' worst.

Like people have said, anything which hinders gameplay is definitely bad. Not being able to complete a level and having to restart it, for example. Or having to restart the entire game, if it's that kind. Also anything which deletes or messes up save data is super bad as well. Those are the things which should be fixed.

Fun glitches can be things like obtaining an item or weapon before you're supposed to and seeing what kind of things you can do with it, such as for example obtaining the airhship very early on in Fina Fantasy VI and being able to visit everywhere you're not supposed to have access to. This is with the caveat it doesn't permanently mess up the game if you don't save it. Other fun harmless bugs are things like visual glitches, graphics getting scrambled or popping up in places they're not supposed to.

Sometimes I wonder how many people are bothered by glitches though. Minecraft is one of the buggiest games ever created, and just look how popular that became.

Oh I have another least favorite kind of bug, from a developer standpoint: Bugs that don't show up in the editor but only appear in final builds. So testing it is a huge hassle. Because everything works fine in the actual editor. And then you go to play it and BAM, doesn't work.