You have Kanban board and there are no free slots in “In Progress” column. It means new stories should not be started — you’ve reached the limit. Developer comes and says “I know that there are no free slots, but I want to start this new small user story, since all the people are already working on the items and my help is not required”. You might think “Hmm, indeed we are moving forward with a good pace, this small story should not take much time…” and you say “OK, John, take it. I don’t think this would be a problem to exceed the limit right now.”
Two hours later Pete comes to you with a similar question: “I just completed my user story, testers will take it soon and I want to start something new.” You say “Well, we just took one story above the limit…”. Pete replies “Please, boss, I don’t want to sit and do nothing, I want to add some value, I am so much into work today!” Can you say no? I bet in most cases you say something like “OK, let’s do it!” to not discourage your teammate.
What happens then? Testers find some bugs in just implemented user stories, but there are no free developer to fix them. John and Pete are working on new stories. They may switch to bugs, but they’d lose focus and you have 2 stories in progress that can’t be passed to other developers easily (it will take time to make the transition and it looks like waste, isn’t it?). As a result, you deliver some stories later. Yes, later!
I understand that it is very hard initially to say something like “No, you’d better go and read something interesting about new technology. We can’t start new story right now, because if testers find bugs in some running stories, we should fix them ASAP, so we need a free developer”. Moreover, you should train your team to not ask such questions. WIP limit is a rule that should be followed. You may change it, but not arbitrarily. This change should have something behind (better productivity, more developers, etc). And this change should be discussed in retrospective meeting or during a Kaizen event.
I should say it is really, really hard to accept the fact, that sometimes developers and testers should do nothing (I mean do not start new stories when they are free). But it’s absolutely required to do so, otherwise pull system will not work.