The top two solutions are actually pretty simple:
F5/Refresh: about half the time simply refreshing the page will resolve the problem. The F5 key, or refresh, tells the browser to reload the entire page, which causes it to once again check the items in the cache for any updates. (It's possible that it also ignores the cache, and simply re-downloads anyway.)
Clear the Cache: about 90% of the remaining instances can be resolved simply by clearing the cache. Depending on your browser, clearing the cache often does more than simply deleting the files. Most caches have some sort of indexing system to speed up access to the files within it. Occasionally, it's not the files that are at fault, but the index itself that has become corrupt. Clearing the cache also resets this index as well.