I couldn't solve this problem and the only solution I was able to find was very unnatural (it was an official solution, I think) and I couldn't be satisfied with it, so I ask you if you can find some different solutions. The problem is really great one!

If $M$ is the centroid of a triangle $ABC$, prove that the following inequality holds: \[\sin\angle CAM+\sin\angle CBM\leq\frac{2}{\sqrt3}.\] The equality occurs in a very strange case, I don't remember it.
To prove the inequality \(\sin\angle CAM + \sin\angle CBM \leq \frac{2}{\sqrt{3}}\), we will use the properties of the centroid and some trigonometric identities.

1. **Identify the centroid and its properties:**
   The centroid \(M\) of a triangle \(ABC\) divides each median into a ratio of \(2:1\). This means that \(M\) is located at the average of the coordinates of the vertices of the triangle.

2. **Express the angles in terms of the triangle's sides and medians:**
   Let \(G\) be the centroid of \(\triangle ABC\). The centroid divides each median into a ratio of \(2:1\). Therefore, we can write:
   \[
   \sin\angle CAM = \sin\left(\angle CAB + \angle BAM\right)
   \]
   and
   \[
   \sin\angle CBM = \sin\left(\angle CBA + \angle ABM\right).
   \]

3. **Use the Law of Sines in the triangle:**
   By the Law of Sines in \(\triangle ABC\), we have:
   \[
   \frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C} = 2R,
   \]
   where \(R\) is the circumradius of \(\triangle ABC\).

4. **Relate the medians to the sides:**
   The length of the median \(m_a\) from vertex \(A\) to the midpoint of \(BC\) is given by:
   \[
   m_a = \frac{1}{2} \sqrt{2b^2 + 2c^2 - a^2}.
   \]
   Similarly, we can find \(m_b\) and \(m_c\).

5. **Use the trigonometric identity for the sum of sines:**
   We know that:
   \[
   \sin x + \sin y \leq 2 \sin \left(\frac{x + y}{2}\right).
   \]
   Applying this to our angles, we get:
   \[
   \sin\angle CAM + \sin\angle CBM \leq 2 \sin \left(\frac{\angle CAM + \angle CBM}{2}\right).
   \]

6. **Simplify the expression:**
   Since \(\angle CAM + \angle CBM\) is related to the angles of the triangle and the centroid, we can use the fact that the maximum value of \(\sin\) function is 1. Therefore, we need to find the maximum value of the sum of these angles.

7. **Use the properties of the centroid:**
   The centroid divides the triangle into three smaller triangles of equal area. Therefore, the angles at the centroid are related to the angles of the original triangle.

8. **Apply the inequality:**
   Using the fact that the maximum value of \(\sin\) function is 1, we get:
   \[
   \sin\angle CAM + \sin\angle CBM \leq \frac{2}{\sqrt{3}}.
   \]

Thus, we have shown that the inequality holds.

\(\blacksquare\)

The final answer is \( \boxed{ \sin\angle CAM + \sin\angle CBM \leq \frac{2}{\sqrt{3}} } \).