The most common runtime error is network connection timeout that happens during uploading a large file over a slow network. The only solution for this is uploading the file using a faster network - for example, you can upload the file from a lab computer on campus. Note that this error is not caused by some file size limit set on the server.
Another runtime error happened a few times recently when some users tried to copy&paste text from an MS Word document to the web editor. We haven't got around to investigating this yet, but it is likely due to some special character encoding that is supported by MS Word but not by the web editor.
And of course, there are runtime errors that are caused by program bugs. If you think it is a bug, please report it in the forum.