wwh writes: I remember reading that the phrase was based on mules' propensity for balking at crossing bridges. The point of the phrase is that dull students couldn't get across the bridge. It separated the bright from the dull.
I don't see how the phrase got me mean a memory aid.

we do seem to have something of a contranym here (stumbling block vs. memory aid) -- it is different to other examples of enantiodromia we've talked about in that it seems to have developed from the original L. differently across different languages.