I was about three days late with my second dose and... mostly, it was just irritability and other mood related things that happened to me. Wasn't very pleasant for me or those around me, but that is the worst of what could happen from missing it or being late on getting it. It won't kill you (well, unless you experience depression and suicidal tentencies which is entirely possible for some - if that happens, SEEK HELP).
However, after you have been on it for a few months... and THEN you start missing doses, you COULD start experiencing withdraws because your body will no longer be providing its own endogenous Testosterone at all. Like it or lump it, but this hormone IS classified as a narcotic for a reason and our bodies respond to its absence just the same as any other narcotic substance.
So best to not be messing around with missing them or being late consistently if you can help it. A couple of days is OK, but not longer than that unless you have absolutely no choice in the matter (which may well be the case for me after my next three doses as I'd lost my insurance and am still figuring out how to continue my T without breaking the bank... ugh).