Allowed where the “prejudice” can be balanced by costs paid by the amending party to the other side (r. 26.01)
If it is really late such that the trial would be hugely delayed/prejudice is too big then court won’t allow
Motion to strike (r. 21): If pleading is fatal in sense that even assuming the best case you can’t make out your claim then court will strike it out (this is rare, only where no cause of action is found, because courts don’t want to strike out plaintiff’s right to have a chance in court)