At the beginning of the movie they show the doctors operating on Ripley, removing the alien from her. Since Ripley's blood is now acid their surgical instruments should at least smoke in the same way that Call's knife does when it goes through Ripley's hand. However, nothing, no smoke.
I asume that Concentrated Acid doesnt burn everything and some metals can withstand it , they did do another couple of Riplys as you see in the movie, so they did know that she whould have acid for blood and they probably made the instruments out of a stronger type of Metal.
this isn't really a slipup.
Keep in mind that Ripley isn't 100% xenomorph, at that point in the series she only shares -some- genetic traits with the xenos. Therefore he blood isn't 100% acidic. proof of this comes in two forms: 1) when she cuts her hand to use her blood to short out the electrical system to her cell (notice that she had to punch her way in first) and 2) when she uses her blood to melt a hole in the porthole in the betty to suck the hybrid out into space. Her blood didn't burn the glass imediately, it took a little time.
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