From what I know, this error should only occur when you have a mod that requires an expansion pack that you don't have installed. I'm not sure why I'm getting it, as I have ultimate collection installed
Anyway, the problem is that I get this error every time my Sim returns from work. When I click 'reset', I can carry on playing as normal (after the sim kind of freakishly jumps out of the car, but that's besides the point) but, because it cancels the action to return from work, no paycheck - or promotion - is received. Anyone know how to fix this or the most likely offenders?

Have you run the Hack Conflict Detection Utility from www.simwardrobe.com?
Have you checked that you have the correct version of your mods for your game configuration?
Have you checked for mods that are obsolete?
My understanding is that a bad gosub error is often the result of an out of date mod, so this is definitely worth checking out. Do you have Cyjon's Smarter EP Check? If any mod need this and you don't have it will give you this error.

As far as I know Bad Gosub Tree Error is an error you get when a mod is searching either for an EP or a file that you don't have. Since you have all EP's I will assume you installed a mod and didn't notice the blurb saying 'please go and grab this or my mod wont work properly'. Trust me it's easily done. I did that myself the first time I installed No Sim Loaded, I didn't see that it needed Smarter EP Check. it's not the only mod that requires something else to work though.

Running the HCDU won't help that. That is simply to show possible mod conflicts. Generally it shows you mods that are working just fine. Nor is it a pop up you get for an out of date mod. If you are running the game with testing cheats on I think that would be more inclined to show an error rather than bad gosub tree.

Now you have to figure out what mod is asking for an add on file. First thought is one related to work or cars, but they may just be a coincident. Run NSL without Smarter EP check and you get glitched births so it may not seem connected to work or cars at all.