When an item stack is placed in the third slot of the hot bar, the numeral indicating the number of items in the stack may occasionally flash or change colors randomly.
What I expected to happen:
Killed a pig and picked up three porkchops; expected it to display a white numeral '3' in the hot bar
What actually happened:
The numeral '3' flashed in a rainbow of colors continuously. Also did this for chicken feathers in the same slot. Had no troubles with this by placing the affected items in other slots.
Since then, reproduction has been limited to only occasional flashes of random colors, not the continuous rainbow of colors I originally produced. However, I have reproduced this in both survival and creative mode games. Further testing in creative mode indicated this only occurred with the first numeral in the hot bar (proceeding left-to-right), regardless of slot. Reproduction in various worlds was hit-and-miss. Was never able to reproduce this with double-digit numerals.
Steps to reproduce:
1. Open a creative mode game on a standard world (not superflat)
2. Clear the hotbar of items
3. Place a stack of three items in one of the hot bar slots
4. Walk around on the ground, observing the numeral