There are two issues here – one is a momentary nuisance, the other can be a real pain.
Loading the Wrong Campaign
If you switch between campaigns a lot, sometimes Roll20 loads the wrong one and you might not notice immediately.
Say you are working on your D&D Campaign, and you exit it and load your Star Wars campaign. But your D&D campaign gets loaded instead. So you exit it and load up your D&D campaign – and again your Star Wars campaign loads!
I don’t know why this happens, but I’ve only seen it (so far!) when switching from a Sandbox campaign to a normal campaign.
My advice if this happens is just to wait a few seconds and try again. It never persists for long.
This problem is more serious. When working in the Custom Sheet Sandbox, it sometimes happens that a specific character gets corrupt, and their sheet stops working properly.
This can manifest in a few ways. Sometimes the sheet seems to keep using old code, so any changes you make don’t seem to be working.
This has happened once or twice on every sheet I’ve worked on. I suspect it’s something to do with how often the sheet’s code gets changed. This just doesn’t happen very often in most normal campaigns, but can happen dozens of times in a single session when creating or updating a sheet.
It can be very frustrating because it’s not obvious it’s happening. You might just think there’s something wrong with the code you’ve just written, spend some time trying to fix that, and then realise your code is fine, it’s the character that’s faulty.
My advice here: when you notice something isn’t working right, and it should be working, create a new character and see if that fixes it.
This is a frustration that happens once in a while, but it’s not a big enough issue to ruin the experience of using the sandbox. The sandbox is still far leaps and bounds ahead of the alternative, so keep using it.