Continuity mistake: When Mina stabs Dorian Gray the last time, the sword does not come out of his back. After she stabs him and Dorian turns around as Mina is pushing him toward the wall, you can see that she put nearly the entire length of the sword through his body right below his heart, yet nothing shows out the back side. (01:03:00)
Continuity mistake: During the brawl in Kenya at the beginning of the film, one of the goons is seen throwing knives at Alan which seem to pin him to the wooden pillar; yet in in a previous shot just before Alan slams the table onto one of the goons, the knives are already embedded in the pillar.
Continuity mistake: When The Phantom tells Quatermain to drop his gun, Quatermain drops it. It cuts and we see the gun landing with the handle pointing out. The way Quatermain dropped the gun the handle should be pointing in. The gun could not have changed angle that quickly when watched how it is dropped. (00:24:00)
Continuity mistake: When Quatermain hits the man to the ground before breaking a bottle on his head, you can see a shotgun on the floor next to the man. When it cuts, the shotgun is gone. (00:10:00)
Continuity mistake: When the League are at Gray's home and the Phantom interrupts, he tells Quatermain to put the gun down. The gun starts falling pointed to the right but a close up shows it landing pointed to the left, but it never flipped. (00:24:00)
Other mistake: When Sawyer tries to take a shot at the Phantom just before he escapes from Dorian Gray's attic, Sawyer fires once, but two impacts are kicked up from the dusty floor. And the impacts come in to rapid succession for a manually operated rifle, even with a crack shot using it.
Continuity mistake: When the man Quatermain wounded with his shotgun swallows the poison pill, you can see a lot of people close behind the man. When it cuts, everyone has moved a few feet away from the man. (00:11:25)
Continuity mistake: When the fourth attacker is brought before Quatermain he poisons himself. He falls on his back with his right arm stretched out behind him, but when the camera angle changes the arm is lying sideways. (00:11:30)
Continuity mistake: When Quatermain hands the rifle to Tom Sawyer, both hammers are down, yet without cocking either of the hammers Tom fires two shots. (00:46:10)
Answer: I haven't read this book of Wilde, but I think alterations were prompted not by copyright issues but because this conveniences the "league" storyline.