Background:
When one upvotes a comment or submission, the up-arrow disappears right away, but there is a bit of network lag before the upvote is actually sent to the server. On some networks, this lag can be significant. Lag on my network has caused me to sometimes upvote an item, close the window immediately, come back later, and then realize that the upvote wasn’t counted. This is more of a problem for submissions, where not only does the submitter not get credit, but the submission also fails to get added to my Saved Links page.
The only way to check whether an upvote has been received is to refresh the page, but that also has the effect of canceling any upvotes-in-progress that have not finished, and requires one to remember which item you upvoted.
My current workaround, after I upvote something, is to leave the tab open, view other pages for a while, and close the tab when I come back to it later. However, this has the disadvantage of me having to remember, when I return to a tab, whether I left it open because there was still something to read on it or simply because I was waiting for an upvote to finish.