Whilst the "best practice" is to inject before eating, this is not always possible. For example, if you are eating out and don't know if you will like what you have ordered. I don't think it is not going to kill you or lead to any major complications if you sometimes inject after eating.
Fast acting insulin does not act immediately and the speed at which it acts varies from person to person. Food is not digested immediately ... and different food are digested at different rates. In an ideal world (and with a healthy pancreas) the insulin peak should match the food peak to avoid any BG spikes (and lows).
With some experience you may start to notice foods which appear to take longer to digest resulting in a BG spike. Some people split their insulin to match the later spike. I would say this is Advanced insulin injecting which few people do at the start of injecting and many people never do.