The Messerschmitt Me 262 'Schwalbe' (Swallow) was, famously, the world's first operational jet-fighter. It was a formidable aircraft, fast and - with a typical armament of four 30mm cannon and up to twenty four rockets - could pack a powerful punch. However, it suffered from numerous problems. Technical problems first with the engines, then with the landing gear, caused major delays. Then political interference (including from Hitler himself) caused further diversions; Hitler and co. wanted the Me 262 to be a bomber, resulting in resources being diverted into developing what would become the 'Sturmvogel' (Stormbird) variant. Ultimately, although the project had begun in 1938, the 'Schwalbe' didn't enter service until July 1944. By then there were fuel shortages (real or imagined) and a severe lack of pilots sufficiently experienced or trained to utilise the fighter to its full potential. Numbers were never enough, and it seems likely that it was too late to make a real impression anyway.
I have seen it said more than once that if the Me 262 had been available earlier it could have been a much greater success, even turned the tide of the air war. I'm not sure.
I hope you can forgive a bit of counterfactual theorising. The airframe design was sound pretty much from the off. Let's assume that the BMW 003 engines worked properly, so the airframe didn't need to be redesigned to take the replacement Jumo 004 engines. That takes us up to November 1941. Four months more for redesigning the landing gear. So we're perhaps looking at March or April 1942 for the Me 262 to enter service. More pilots, more resources. Admittedly the MK 108 cannon wasn't available at this date, but there were alternatives (and I imagine the aircraft could be pretty easily upgraded once the MK 108 was in service from Autumn 1943).
So, could our hypothetical 1942 'Schwalbe' have really turned the tide of the airwar? Or was it already too late? Perhaps captured intelligence and/or examples might have allowed Allied projects like the Gloster Meteor to have advanced more rapidly. I daresay Allied countermeasures would have been evolved, though of course the Germans would have responded.