In Serbia, a formation of American P-38 Lightnings flying from their base in Italy attacked a ground column of what they though was German infantry in Serbia, only to be hastily intercepted by what they thought were BF-109's from a nearby Luftwaffe instillation. As it turned out, they were Soviet troops and the interceptors were Yak-3's, and a dogfight ensued before both sides withdrew after about 5-10 minutes, with both American and Soviets suffering casualties. The incident was quickly forgiven and forgotten, but it's the only example I can think of where the two sides clashed accidentally.

(Only article I could find was here)

There was a fairly serious friendly fire incident on 18 March 1945 involving the 359th Fighter Group operating in the Berlin region. After performing their part of the escort leg, elements of 369th FS followed two bogies to the airfield of Zackerick, which unbeknownst to the P-51 pilots was occupied by the Soviets. The American pilots misidentified the airfield's Yak-9s and La-5s as Bf-109s and FW-190s respectively and proceeded to strafe it. The result was a complicated melee on the ground and in the air where the Americans shot down nine Soviet aircraft and destroyed four more on the ground with no losses to the Americans.