Put the summary of the bug you're having here
What I expected to happen was...:
for all the blocks to appear
What actually happened was...:
it will sometimes load but then crash anyways
Steps to Reproduce:
1. open up one of my games
2. i can sometimes play a bit
3. but then it crashes anyways
I hate to break it to you Alex, but you provided insufficient information to fix this bug. What would be very helpful is if you attached your crash report. If you are on Windows, then to get it, go to your Windows Explorer (the Yellow Folder Thing), type "%appdata%" in the top bar, click ".minecraft", click "crash-report", and there should a file in there for every crash you had. If you are on Mac, look it up. Then, attach at least one to this bug report.
Hope I was helpful!