I don't think the lid can fall in on a 15, but be careful with the screw. This it shouldn't be a particular worry, but in theory a lot of thigns can happen. There are quite a few helpful blogposts on 15s; some of the nicest are on this blog;
here and
here, yet another one
here.
On these I have had three issues that needed a few days of tenative care to be sorted out. I have had the stitch length stuck in one position regarless of the stitch length lever position. I inspected as well as I could, turning the hand wheel and observing parts. I tried to follow each part from the stitch length lever to the feed dogs. There are quite a few involved. The horizontal rods can need attention, and lots of good light to get at the tiny holes and joints where oil needs to go.
The other one was the sticky needle bar, and the third was a part behind the faceplate needed a bit of extra oil to smooth out. It can take a day or two for the oil to seep in and do it's work. Oil a bit everyday for a while if it's not running optimally. They need oil once or every other week, and more if you use the machine hours a day. Once you have a sticky or noisy machine it can take a bit more effort to get up to standard again. It's usually not difficult, you just have to do the job thoroughly even if it seems a bit tedious or even unnecessary (most of the time it's not).