Yes, some PCMCIA (Personal Computer Memory Card International Association) cards can be incompatible with older slots due to differences in the standards across various generations of PCMCIA technology. There are three main types of PCMCIA cards and slots:

Type I: Primarily used for memory cards.
Type II: Commonly used for I/O devices like modems and network cards.
Type III: Thicker cards used for devices like hard drives.
Additionally, PCMCIA evolved through different versions:

16-bit PCMCIA (PC Card): The original standard, often referred to as PC Card. This version is compatible with 16-bit bus architecture and has limitations in terms of data transfer rates.

CardBus (32-bit PCMCIA): An updated version that is backward compatible with 16-bit cards but not forward compatible with 32-bit cards in 16-bit slots. CardBus cards operate at 32 bits and have faster data transfer capabilities.

ExpressCard: A later standard that is not compatible with older PCMCIA slots. It uses PCI Express or USB technology and has a different form factor.

Key compatibility issues:
16-bit vs. 32-bit: Older 16-bit slots cannot support newer 32-bit CardBus cards. However, CardBus slots are usually backward compatible with older 16-bit cards.

Voltage Differences: Some older cards may use 5V, while newer slots/cards may operate at 3.3V. Using a card with the wrong voltage could lead to issues or damage.

Slot Size: While the physical size is generally consistent across types, some Type III cards are thicker and won’t fit into Type I or Type II slots.

So, if you're dealing with older equipment, it's essential to verify the PCMCIA card and slot types to avoid compatibility issues.