Is it possible to have a “bad” viva?
There are lots of general expectations about the viva process. A reasonable expectation for the duration is two to three hours. There are outliers: it’s possible to be finished in less than an hour, but it’s not a possibility to be hoped for. My viva was four hours and I once met someone whose viva was five hours!
I don’t think my viva was “bad” but can imagine that for another person four hours would have felt like an awfully long time.
Maybe there are certain questions that would feel bad to receive. Perhaps a particular focus by an examiner would be unwelcome. There’s a very remote chance that an examiner could approach the viva with the wrong attitude: looking to find problems or to show off their own knowledge and experience.
That would be objectively bad and it’s very unlikely, thankfully.
Most ideas of a “bad” viva are subjective: you have concerns about what could make your viva “bad” for you. If you can name those concerns then maybe you can do something about them.
For example, if a “bad” viva would be one where you forgot things, then you could take steps in your prep to help in case that happened. If a “bad” viva focussed on a particular topic, then you could do extra reading in preparation, or take time to rehearse more for talking about it.
If you have an idea of something that would make your viva “bad”, first check to see if it’s at all likely. Knowing that it probably won’t happen could be enough to help – but if not, consider what steps you could take to help yourself.