You are running out of memory (heap space) not account space.

After you run the test, check the account (`userInfoAddress` in your test) with this command `solana account %account_pubkey%` (local validator must be running) and you will see that the account bytes returned have a lot of unused space (the padding 0s at the end). So there is still space left in the account.

```
        ...

77c0:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
77d0:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
77e0:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
77f0:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
```

I have run your test and consistently get `496 friends` added, I cannot reproduce the situation where "it throws this error randomly", so my answer is based on my results where the behavior is consistent. Although it still runs out of memory faster than you would expect.

Why it's running out of memory is because when you use `Vec<T>.push()`, it will reallocate memory in a way that is best suited and performant for most use-cases, and since memory reallocation is expensive it often just doubles the memory needed by the `Vec<T>` just so that it will not have to do a new reallocation too soon. So often your `Vec<T>` that by your calculations is, say, 100 bytes, will actually take up up to 200 bytes of memory.

So given that `Vec<Friend>` is `33 bytes` * `number of friends`, the program in my case consistently fails after it added `496 friends`. At `496 friends` it  represents `16368 bytes`. At the next `.push()` it will try to double the `Vec` size which would now be `32kb+` bytes.

Given that Solana programs are limited to heap size of max `32kb`, our `Vec<Friends>` + other variables in memory easily go over this limit and an error is thrown.

You can see these changes in memory allocation if you add this code to `pub fn add_friend()` after the `.push()`

```
let capacity = ctx.accounts.user_info.friends.capacity();
let size_of_friend = std::mem::size_of::<Friend>();
msg!("Capacity: {}, Size Of Friend: {}, Size: {}", capacity, size_of_friend, capacity * size_of_friend);
```

Notice how `capacity` of the `Vec` changes, with doubling often when it reallocates memory. [These are the logs from my local machine.][1]

There are different ways of manually allocating memory for a `Vec` in Rust, such as `Vec::reserve()`. 

PS: I appreciate you adding the git link to reproduce this error.


  [1]: https://gist.github.com/serbangv/84220eded948e4c363cb99bb997c00f5