^^^ Clearly +1 +1 +1 for all above.
Make sure all equipment, spoons, alcho-swops, micro-filter, even if there's still some saline solution left in that last vile - break a new one.....but I'm sure you/we all do this right?
In terms for a missed shot - depending on which vein was used as to how infectious (if it does at all) it will be.
At the very least there will be a lump, it'll be tender and possibly itchy and bruised. If any of these change either by the swelling going up or down your arm, itchiness moving or, if it stays constant if it doesn't improve in any later then a week and a half ...... go immediately at the very least to your GP! Generally a standard course of anti-biotics will fix it, if the infection gets worst and you don't get it seen to it can quite can quite easily become systemic and once it travels, it's not "just a case of anti-biotics" anymore!