I've been warned to go carefully if I have having extra injections. Yyou can get problems with the insulin stacking up and making you hypo a bit later. I've also been told not to have injections less than 2 hours apart, thought I presume that depends on the type of insulin. Mine is novorapid.
I find if I do have an extra meal/snack that I need a lower dose than usual for it. I;ve recently got a meter which caculates it for me, taking into account the previous meals dose and it has helped with this sort of situation. Before that there was quite a lot of guesswork.
The other option is a meal/snack without carbs. Things like cheese, cold meats, eggs, salad, nuts are all options without the carbs.