It is very common, especially when a team is new to Scrum to talk about extending the Sprint. This usually happens for one of two reasons:
“We didn’t finish the scope of the sprint”
If a team is new to Scrum, more often than not, they still have in them the Waterfall mindset. We cannot move to the next box before everything from this box is done (see a box as technical/functional design, development, test).
The team sees each Sprint as one of those boxes, and the previous “way of working” prevents them from seeing why they would close the sprint.
If your team is in this phase, try coaching them that the sprint is not a BOX of Scope, it is instead a BOX of Time.
“We didn’t reach the goal of the sprint”
“If the goal of the sprint is to reach the goal, why should we close it if the goal has not been done yet?” I have heard this one once, how would you reply?
For me the explanation went something like this:
You are correct. The main goal of the sprint is to reach the goal, and at the same time, the Sprint is a time-boxed event. We closed it with a "failed" goal because we are transparent. There is no problem in failing, as long as we don't fail twice for the same reason. Now we go into our retrospective and reflect on what could have gone better and what we can do in the future to make sure it never happens again.
And this, for me, is why we should never extend Sprint for any reason. We need to have the courage to be open and show when we fail. We need to be smart in finding new ways to not fail again.
Now the disclaimer part.
Should you, for any reason, extend it for 1 hour or 2 without making it a standard? Well, yes. Take it with a grain of salt. More importantly, check how much that would impact the morale of the team. The GODS of SCRUM will not rain down on you with a vengeance just because you took 2 more hours to close a sprint. And if they do, that’s their problem. You should know your team better and what is best for them.