In hindsight, it looks like in both cases the first-day pop was achieved by constraining the shares available to buy.
Both stocks have quickly lost most of their first-day pops:
https://www.google.com/finance/quote/CART:NASDAQ?window=5D
https://www.google.com/finance/quote/ARM:NASDAQ?window=1M
Not a good omen for IPOs on the pipeline. Sigh.