If you have cost reducers that reduce a spell by MORE GENERIC MANA THAN IT COSTS, and the spell can also be cast using phyrexian mana(either naturally or by using something like the Defilers or Krrik), then the game will glitch and think you cannot cast the spell when you actually can and you will lose priority(if you have nothing else to do).
You will also even lose priority on your opponent's turn if it's an instant!

Examples:
Temur Battlecrier x2 + Phyrexian Metamorph (generic cost reduction of 4 vs Phyrexian Metamorph cost of 3 generic and P/U)
ANY cost reducer + Surgical extraction (generic cost reduction of 1 or more vs Surgical cost of 0 generic and P/B)
4 instant/sorcery cost reducers + Tezzeret's Gambit (generic cost reduction of 4 vs Gambit's cost of 3 generic and P/U)
3 instant/sorcery cost reducers + Dismember (generic cost reduction of 3 vs Dismember's cost of 1 generic and P/B P/B)

This does NOT bug when the spell has a coloured pip AS WELL as the phyrexian mana, it seems to only bug when the spell is effectively "free" due to phyrexian mana (as well as there being extra cost reduction obviously)

Example where it does NOT bug:
3 sapphire medallions + Jace the perfected mind
3 cost reducers + Tezzeret's Gambit (Cost reduction = generic cost, will not bug)
1 cost reducer + Tezzeret's Gambit (Cost reduction < generic cost, will not bug)

All of these examples have pictures that go with them
Video proof only exists for the Gambit and Surgical bugged examples, but all of these are easily testable.

The screenshots of the bugged examples are taken directly after all the conditions are fulfilled, which usually translates to the game skipping past the end of Main 1 and moving to combat, which is what you see in them all.
(The Dismember screenshot is taken on my opponent's turn because I found Dismember on Main 2 so when the conditions were fulfilled instead of passing to combat it passed directly to my opponent's turn and I didn't have a chance to catch a good screenshot)