hit a snag

to "hit a snag" means to come across difficulties i think.

To encounter problems or difficulties - you may have heard of the phrase 'it was plain/smooth sailing' which means everything went ok. 'hit a snag' means there where problems which prevented everything going smoothly to plan.

Literally, a snag is something that gets in the way, specifically the jagged edge of a rock or a tree or something. So if you were pulling something by a rope, and it got caught on a tree, you would literally 'hit a snag' which would delay progress.

Hence the reason it's used to indicate running into an unforeseen problem or delay.

You can also have a snag in your sweater or stockings.

The tearm may come from weaving and sewing. Early sailors spent as much time mending rope and sail as they did fording reefs and log-clogged rivers so it's probably impossible to chicken this egg.

"Snag" can also mean grab. You can "snag great tickets" for an event, for instance. In this case, you would be the rocky shoal.