I had this issue happen to me, and i think its simple to reproduce. (have not tried it but most likely is) I also think this happen to me with the same steps awhile ago but did not report.
Since above comment said post back to the original user i decided to post it here instead of creating new bug.
Don't think it matters but I am on Win7 and on Firefox.
I had multiple tabs open. The steps are as follow.
1: Have two or more tabs open.
2: One of the tabs has to be in the gym.
3: On the other tab, fail arson crime. (which will send you to hospital) This possibly happens on other fails as well that sends you to hosp and maybe even jail.
4: On the other tab that is on the gym page, try training.
If dev thinks this is separate issues, let me know and ill create new one if needed.
While this is minor issue and users will most likely figure out why, its confusing especially after someone closes the tab after step 3 and comes back later for example and try to train at the gym ( that page has not been refreshed so shows grey page instead of white.
There might also be other scenarios that this can happen and by fixing this may also fix those.
http://prntscr.com/bwg7m1