Cut chain too short

Depending on the chain, you may need another connecting pin to put links back on. It's not a big deal.

I've never been able to get the pin back in. However, I've had really good luck with the SRAM powerlink . Just pick the one for your chain size and it makes chain removal easier (just watch the video before assuming you know how to do it).

Another option that I've done is to remove an additional link from your chain and then in combination with the chain you removed you can usually splice in via two separate pins enough chain to remedy the mistake. It is much easier to explain in person rather than typing.

If its just one link short then get a chain connect (Wipperman Connex, KMC, SRAM, etc.)
If its more than one link, get the required lenght of chain, buy a virgin chainpin at the local bike shop and pop it in.

If you need one pin, buy a SRAM powerlink or whatever.

If you need more than one pin, buy 2 SRAM powerlinks or whatever, buy a chain, cut the chain to 5 or 3432 or 4323 links, then stick between two powerlinks. Then attach.

Powerlinks also make for easier cleaning of the chain off the bike. Just don't let the go down the drain.